Keeping a mini and a big horse together can work depending onthe personalities of everybody. For example, my filly Misty has gotten out into the big pasture with Jake and Elroy(my qp and tb) and they were fine untill I got out tp the barn(about 20 min away). They could be kept together if I needed to. But had my little gelding Copper gotten out, somebody would have gotten seiously injured(problably Cop). So it could definatly work depending on the personalities of alll the horse involved. Or you could even keep the mini in a pasture/pen that is sharing a fence with your big horses if you are not completely comforatlable with keeping them together.
As for getting a mini for your child, if you truly want a mini, please get a B size. YOu could also try a small welsh pony or shetland. Yes shetlands have a reputation for trouble, but trained properly they shouldnet be any more trouble than any other horse/pony. If you do go the mini route, I would be afraid that by the time she/he was able to ride, they would have already out grown the little horse :/. Well, whatever you choose I am sure you will love as will your baby. :)
~Do Your Best, Prepare For The Worst, Then Trust GOD To Bring The Victory~Proverbs 21:31