Regardless if there are older kids making use of the trampoline with younger children, you can't rely on them to watch the younger ones.
6-8 decent lifts gets the best strength developments and vertical carryover.
Standards For Immediate Methods Of how to jump
When you land stay in the same position for a few seconds, and then step back on to the box and repeat.
Imagine, if you're raising to fatigue, you're not activating all the muscle fibers you will be.
No-Fuss how to jump higher Advice - The Basics
When you land stay in the same position for a few seconds, and then step back on to the box and repeat.
Heck, even go for elbow and knee caps while you're at it.