I have no personal experience in the matter, but my guess is that the monies would be placed in some kind of trust, for which the parents would be trustees. I would also guess that the parents would need to seek some kind of authorisation (from a court or the social services) before using babies in films/ads etc. As far as I know, the lowest legal age to work is 13 or so, the age when children start delivering newspapers, and so I think that babies who "work" must do so with permission from someone over and beyond the parents... I stress that I am only taking an "educated guess"...