[Baby Buddy] Hide child selectors when only one child exists

I suspect a decent number of Baby Buddy users, myself selfishly included, only use Baby Buddy to track the behavior of one child. They may only have one child, or if they have multiple children, their ages may be disparate enough where the older child or children don't need to be tracked anymore. In those kinds of scenarios, I suggest hiding the child selection element when adding entries. For example, if I'm logging a diaper change, but Baby Buddy only has one child stored in it, then the "Child" field on the form "Add a Diaper Change" wouldn't appear and the child is assumed to the ID of the only existing child. This shouldn't affect the API however as that changes some of the validation assumptions; the API should always require child IDs when applicable. This is different from https://github.com/babybuddy/babybuddy/issues/836: rather than a user preference, this is based solely on what data is present in Baby Buddy.