There is no restrictions on buying bitcoin for kids but it will be risky if you put all responsibilities of investment to them in early age because they will have no knowledge at start so they can ruin all invested amount in a second. One should think that businesses are not for kids but if someone wishes that their children never face any difficulty in future then they should buy bitcoin or another asset for them and then gradually teach them so when the become teenager then put all responsibilities on them. Totally dependent on bitcoin from an age of kid is not a good idea because they will not take any interest in job if they find bitcoin therefore just teach them but don't give bitcoin in their hands earlier.