The Nationality Law of Laos allows only one nationality. While many Lao and some foreigners think dual nationality is allowed until a child reaches 18, this is incorrect. If a foreign man is 100% truthful in registering himself as the father with the Lao authorities, his child will be allowed to remain in Laos only on the basis that the child is accompanying his father. The father must of course have permission to remain in Laos. If a child is registered as English, French, etc., it needs a visa stamped in its national passport (and foreign passports are not always easy to get quickly after birth).
I know of only one foreign man (French) who has gone the legal path. He is officially married to his Lao wife and has two children. He speaks Lao at home with his family and his children know very little French but are French and have French passports (obtained quickly with the help of the French Embassy). He pays each year to get each child's stay in Laos renewed. He also has to maintain their French passports. The children are not on their mothers Family Book because they are not regarded as Lao.
I know of MANY foreign Englishmen who are married to Lao women, with or without a certificate to prove it, and have children here. Of those I know, ALL had their wife register the child as Lao by inclusion in the Family Book (to which the husband is excluded). All children are registered with the mother's family name, not that of the father. Once in the Family Book, children can obtain Lao passports, and eventually ID cards, with no problem.
The Nationality Law says the nationality of the father, if known, is the nationality of the children. Thus, all such foreign fathers are breaking the law. This 'illegality' seems to be well tolerated and I know of no foreign father who has been arrested, fined, asked for money, or made to leave the country for failing to abide by the law. Most intend to get an English passport for their child/children eventually, but most do not get round to it unless leaving Laos permanently.
If you are in this situation (and many foreigners are), which path did you take? I am not here questioning the Nationality Law (2004), which is on the internet in Lao and official English translation. There should be a better way of doing things than breaking the law. Maybe there is and I simply don't know what it is. Do you?