I have listed some negative things about living in Mauritius and a couple of them only pertain when comparing to a London or an equivalent large city. I specifically mention London as you have indicated that that is where you are currently. At any rate, these may or may not necessarily be negative for you, but you may want to be aware of ahead of time.
1. It's a remote island in the middle of the ocean. It will take a long time for friends and family to get here if they want to visit or for you to return home to visit them.
You will not be able to get back to London quickly such as to see a sick relative or a there is a death in the family. Direct flights are 12 hours plus getting to the airport 2 hours before departure, travel time to and from the airports, clearing customs, possibly waiting for luggage, etc.
It will take at least 15 hours if you are lucky and everything lines up perfectly and you can get on a direct flight within a couple hours. Worse case scenario, it could take a few days to get a flight as happened to a friend who was visiting and missed his departure because of two flat tyres on the way to the airport. He HAD to get back home quickly, but every flight off the island on every airline where he could get a connection was full in economy, business and first class for 3 days.
There are also not as many options to travel to other locations (or countries) for a weekend getaway as being based in London offers.
2. It's also a small island and does not have an overly large population which can potentially have negative repercussions. Because it seems that most expats/locals typically spend the vast majority of their time in and around where they live, you are much more likely to continuously run in to the same people (which can be good or bad depending on the situation and people).
Word travels fast so you will potentially have your personal "business" talked about and known very quickly amongst your circle of friends on the island or complete strangers as well if "interesting" enough. This will depend in some part on how "gossipy" your friends on the island are obviously, but word does travel fast here.
3. Compared to London, Mauritius has no where near the number of options in terms of shopping, restaurants, bars, night clubs, sporting events, cultural events or activities. If you do not need endless options of things to do or places to go out to then this may not be a big issue as too little/much choice can be a good or bad thing.
4. Mauritius is a third world / developing country in most respects. This in and of itself is not necessarily a negative thing but you will have to accept that there will be power outages ranging from a few minutes/hours to potentially days, the internet runs much slower and the tap water is not the cleanest and does not run 24/7, but if your home has a large storage tank and you don't leave the garden hose on accidentally over night you'll be fine (a lesson I only had to learn once).
The most negative aspect pertaining to this section is that you may or may not get a response from the police or emergency response phone numbers when needed of which I have had first hand account on a few occasions unfortunately. So if you need an ambulance, you will be much better off (if possible) driving to the hospital yourself or having someone else drive you than waiting to hopefully have the emergency response number answered and then maybe an ambulance and/or the police may come to you.
5. Driving in Mauritius takes a bit of getting use to and can be quite daunting for some people at first. People may drive on the same side of the road as in England, but most people in Mauritius do not follow any discernible road rules and the lack of an emergency lane / shoulder on most roads creates for some interestingly intense situations as well.
Tailgating seems to be a national sport in Mauritius as is the local habit of parking in the middle of the road even if it's possible to remove one's vehicle from the road and make use of a proper parking space. I haven't actually found the rule book for these games, but it seems that bonus points are awarded for parking in the middle of the road on either side of a blind curve or just on the other side of a hill.
Overtaking around blind corners and/or while going uphill is also a quite popular game. There is also a shared "invisible" middle lane used by vehicles going in both directions for overtaking that seems to follow first come, first served protocol. I have not been able to find the special "glasses" for sale in any store that will allow a player to see this invisible lane.
It is also important to note that when a vehicle is parked in the middle of the oncoming lane and another oncoming vehicle is approach said parked vehicle, do not assume that you have the right of way just because your side of the road is free from obstructions. No, you would definitely be mistaken and you should yield your vehicle so that the other vehicle (especially if it is a Triolet Bus Service bus) can can pull around the parked vehicle with out having it's progress slowed. You have been warned.
****Section 5 heavily laced with sarcasm for those unable to pick up on it****
6. Things move at a much slower pace in Mauritius than in London. Again, this is not necessarily a negative thing and can be a very positive aspect of living in Mauritius, but you will have to adapt and learn to not expect the same urgency you may be accustomed to in people in a big city or the same hectic and often times stressful lifestyle as is common in larger cities like London.
7. Traffic congestion going in to and out of Port Louis can be TERRIBLE so if you are for instance going to work a 9 to 5 job in Grand Bay, then don't live in Curepipe and vice versa. As a general guideline, if you work North of Port Louis, then live in the North part of the island. If you work south of Port Louis then live in the south part of the island. If you will be working in Port Louis, it doesn't matter too much where you live so I hope you like traffic. If you do not mind getting stuck in two traffic during rush hour then live where ever you like most as it's hard to get away from the traffic regardless at any rate.
Depending on how independent, flexible and adaptable you and your family are, any of the above may or may not be an issue. I personally find life in Mauritius much better than compared to my years of living in London. I love living in Mauritius as I am sure you and your family will too especially as the weather here is at least a thousand times better than in London....