In relation to which type of life jacket the crew would issue to your child, I'm reasonably certain in almost all cases it will be an adult life jacket. There is only a very limited number of infant life jackets, or other flotation devices, on board, and these will naturally be prioritised to infants. The weight restriction mentioned earlier is only as a guide for the crew to know the maximum weight that an infant life jacket can safely support (some babies are huge!) rather than a guide as to whether it should be given to a child.
My best recommendation to you, would be to have a talk to the senior cabin crew member or purser, on your next flight and allow them to guide you with regard to the exact circumstances at that time. I'm sure they would be pleased to assist with a query a little different to the normal kinds of questions they get asked, and would be pleased to put your mind at rest.