I find that tight shots of near-nearborns aren't as pretty as when they are 1 year and older. A 17-55mm or 50mm fast prime would allow for full body shots in very low light.
I think at a younger age the 17-55 would be a splendid all-around lens to capture just about anything with a 50mm prime added for even more low-light ability. You can add an 85mm or longer in a year or so.