Beau says:

Making "family books" (one book for more than one child) is something that we haven't decided exactly how to do just yet Angie. Until there is a better way, you can just set up the profile information to reflect both children ("Bob & Johnny's Book"), and then probably just don't enter things like height/weight.

Most likely when Family Books are available they will work something like this anyway (no growth chart, with combined details/no height/weigh). Hope that makes sense.