one other thought: where would the kittens be coming from? whatever environment they've been in, they will be bringing some of it to you
with a diabetic cat, whose immune system is already stressed, you don't want to introduce anything noxious -- a kitten from the shelter, for example, would have been exposed to a number of things you have no control over -- and in any case, you will want to isolate the kitten(s) from the other cats for some time, for the safety of all, just another hassle to deal with
so much depends on the personality of your present cats, and how much time and effort you have available to deal with the kitten(s) as well as your own furbabies -- at least I hope you make sure the new ones have been vaccinated and altered (I'm very much in favor of "pediatric" altering)
yes, normally I'd say, the more the merrier .. but I wonder -- how old is the daughter who wants the kitten? is this a recent wish, or something she's been wanting for a long time? (I remember promising that I would care for the kitten I wanted, when I was about 7 or 8, but my mother wound up doing almost all the caretaking, broke my heart when we moved and had to leave the cat behind, NOT my choice)