Go to a large hardware store like Lowes and go to the trash bag isle. Look for contractor trash bags. I buy these for our outside trash cans.
The 33 gallon trash bags, think large leaf bags, do fit the outdoor trash cans but just barely. They often come off the edges and don't work right.
SO I buy the 45 gallon large black trash/contractor bags.
These are perfect for storing things in the attic, if you want to add some moth balls or other rodent deterrents you'll need to research that. They keep most insects and critters out.
I think it's very nice of you to save these for your kiddos. I have a lot of stuff from when my daughter was little that my grand kids have used and I still have some of it.
My mom kept rocking horses, toys, clothes, and more in the attic for 20-30 years and they were fine.
I will say that today's materials might not last as long, but they might even be better, than yesteryears materials. I'd make sure they were not brittle and easy to break when you take them out of storage.