I don't recall the exact weight threshold, but airlines are required to distribute smaller life jackets for all children under 40-50 lbs. I would put the kid's jacket on first, then put on my own or, if none is available, grab the seat cushion and head for the exit. Of course, if you're flying solo with multiple small children, hope that a fellow passenger can lend a hand (I'm assuming the crew will be too busy directing traffic and doing other things to hold babies)