We sail with 3 kids. Rules are, when underway, everyone has a life vest on
( except in the cabin.) The adults had inflatables. With passages,or rough seas, we harnessed in, including the kids, under the lifejacket.
Though we sailed from the time they were babies, we only started using harnesses when we were cruising. The youngest always had a lifejacket on, except when in the cabin. This still holds true until he proves he is a better swimmer. ( he is 7 now).We have started letting him sit in the cockpit without one when at anchor. The older two are teens and now just wear lifejackets when under way ( with protest from them and the captain). We now have inflatables for them to make it easier. Harnesses for rough seas and passages.
I would not trust an inflatable with a younger child. Too many variables.
Get one with the hook on the back, handy in more than one instance.
( Moving from dinghy to boat, rock climbing
. My youngest wore one so often it was second nature.
It is best to establish rules that are clear. Be aware that if you wear one as well, you set a good example!
Best wishes. This is just what we have done, Cruisingmom and chief safety officer.