I wouldn't advise you to take any children that young, especially on the weekend when it's busy. We took our 4 year old during a weekday when we basically had the place to ourselves. That was manageable. I'd be worried about an 18 month old getting trampled when its busier with lots of kids jumping around.
For trampolines, that's a big NO. Children under 6 years of age aren't advised to use trampolines due to the forces produced by jumping and landing alone. I believe the toddler area, which is posted 5 and under, is trampoline-free. It is just a foam block pit for them to plop into. If everyone stays in their age appropriate area, and are well supervised, everyone in your family should have fun.
18 months still a bit young...weekends are super busy...
There is a section just for the younger kids, but someone would have to be with him/her the whole time to be safe. Kids aren't very careful of their surroundings when jumping around. If the older kids can help watch the middle aged kids you should be ok. Will probably need some Advil afterwards though for your headache...
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