For really young kids, things like flash lights/head lamps, binoculars, compass, small gardening tools (trowel, hand rake), maybe a simple container gardening kit (this a fun activity and teaches gardening and science in a hands-on way), a small tent, etc. At this age, it's about introducing them to the basic concepts of life and you want prepping to be part of that. So making it normal to have and to use these items is a good idea. Other ideas are simple books that teach about the way the world works in general - we had a bunch of the Scholastic First Discovery books when we were kids and I learned a lot of foundational concepts about the world that way. You might ask what this has to do with prepping but it's important to know how things work if you're going to be able to survive.
For kids older than about 6 or so, they certainly can start having things like a camping gear, tool set (hammer, screw drivers, pliers, wrenches, etc), multi-tool (I remember I bought one of these for my maybe 5 year old brother and my mom didn't think he was old enough yet but he liked it), more advanced gardening supplies, first aid kit and first aid instructional book (we had a children's first aid book as kids and we would practice on stuffed animals because my mom always told us not to do practice on each other - most of my foundational knowledge of first aid comes from these experiences). Depending on what these kids are familiar with, archery, hunting, and fishing gear could be good as well.
For 12 and up, pretty much anything prepper related can be good as long as they are taught the safe and correct way to use it. Classes on different topics related to prepping (first aid, self-defense, etc) might be a good idea too at this age.
I have almost always bought my siblings useful items and prepper supplies rather than toys. I'm getting my 13 year old sister a machete this year (by her request) and often give pocket knives, camping gear, etc depending on their interest and what they want but don't have.