A pair of binoculars will grow with them and last for awhile. No need to buy the top tier glass, this is where I love budget glass with a good warranty like Vortex. My daughter is only 3, but loves going on hikes so while we go on smaller hikes we will stop to take a break/use the binoculars, and I got her a Viper monocular that's a little easier for her to use with her little hands. Just tie a simple lanyard with paracord and she is off to the races!
Other than binoculars, for gear that will grow with them, don’t get super crazy. A decent knife can be had for under $30 pretty easy (I grew up with a Gerber Gator, although there are tons of good and super affordable options like the Mora blades). A decent used pack can be found pretty easily (look at the smaller Osprey or Kelty, they are super nice packs for the money, and used ones are not hard to find). It would be worth it to get good trekking poles as they are going to be usable forever/and are semi-hard to lose just by nature of how you use them. If your kids have social media accounts, don't forget to just take plenty of pics for them to post later if that is there thing. Don't underestimate how important it can be for some kids to post a few pictures of the scenery or animals.
If your kids have an allowance or do things like cut grass for the neighbors, let them buy some gear for themselves. Help them find a good deal and walk them through what to look for. Take them to different stores and flea markets (when they open again) or yard sales and let them take the time to find gear they want to buy with their own money. It’s a huge life lesson to start buying your own gear, and give them a chance to learn what to look for. I'll spend hours of research on gear, give them a chance to explore the options as well. Even if it's just getting them to a store and letting them look at a few options and making up their mind what ones they want to buy (or for you to buy for them). Let them truly decide what they want and then talk with them and help them decide if it's what they want. But don’t pressure them into wanting a specific brand/product if you go that route. My dad was never much of a hunter, but he was a super gear freak with tools. He did the same thing, would talk with me about what to look for, a good price range and then support me if I asked him what he thought about XYZ brand tool. Not in a "that’s a crap socket set" way, but in a "that’s a good deal, but they may not last too long, what if we go around and found you a used Craftsman set for the same price? Would you be interested?" My point is just that we can often lose sight of the fact that we are gear freaks in our own way, find a way to support yours kids own gear choices. To me, that always made it more special. If my dad said it was a good tool and helped find a way to make it more affordable for my budget it was always a win win. I still have some of the first tools I ever bought with my dad because they are just that important to me now.
I guess at the end of all of this, im trying to say, the best investment for gear is probably the time you spend helping them pick out gear. Shoot, get them on the forums looking for gear reviews on the more affordable gear.