Thank you Amy for that great PHOTO and all the suggestions! Kudos to your church for going the extra mile. (I see that you are now offering outdoor, indoor, and online worship!)
Here in Ohio, outdoor worship is super-iffy this time of year.
Two weeks ago we attended a local church's "drive in" worship in our car. They had done this in past summers so they had the radio broadcasting equipment (which isn't expensive). Saw lots of kids bouncing in seats and sucking on juice boxes. (It all felt strange and like a novelty, but I didn't have any kids with me so maybe it was better than nothing for those who do.)
Over the years, I've run hot and cold on "worship bags." I've seen them become a distraction to both kids, parents, and others seated nearby. Yet I've heard parents say how helpful they were. Interestingly, in churches where we did not have them, I don't recall anybody asking for them.
If I had to pick just a few things, I'd only pick ONE actually: the Magna doodle. They keep the kids' attention, don't create waste, ...and the "pencil" is attached with a string Various websites sell a bunch of different brands/types/price points. Buy one to test it as some aren't made very well.
I'm a big fan of LEGOS, but I would not recommend LEGOS in a sanctuary. LEGOS are noisy and bouncy when they are dropped. On your outdoor gravel floor that wouldn't be a problem. Now... if the pastor somehow integrated their use into the sermon/service, I might change my opinion.