I had a lady ask me while I was securing the cabin just before take off, if 'she could wait a bit' before putting her child in the infant belt other wise 'it would scream.'
I can't stress the importance of putting on an infant seatbelt. If the pilots suddenly jam on the brakes while taxiing, your kids gonna go flying (literally) and then it really will be screaming.
I always ask parents if they have one and if they know how to put one on. Like Iguana and Nans have said if you don't know how please ask us!