National Dress for Children

What do you mean by "Khmer National Dress" exactly? Are you referring to krama/sarong/sampot or what you see Apsara dancers wearing at the tourist shows. There are so many variations of traditional Cambodian clothing, you will need to be more specific.

There is no modern day "Khmer National Dress" as such.

Quite right. I am looking for the dancers' dress-do you know where I can find it to buy?

I am not sure where to buy, but most photo/portrait studios have all sorts of traditional Khmer outfits for people to wear for themed photos. Maybe pop in and ask at a few places, I am sure they'll be able to point you in the right direction at least.

You could also try asking at the University of Fine Arts on St.19 behind the National Museum, as they teach Apsara there.