If Twistables don't work for you (my kids had them all twisted out--it was way more fun than coloring!) then I would give each child their own crayons. The 5 year old and 4 year old ought to be able to take care of their crayons better and not break them, allowing for the occasional accident. The 2 year old, no way!
My boys each have their own "art box." My 4 1/2 year old's is a pretty big Sterilite Show Off with a lock-on lid, and has a tape dispenser, stapler, ruler, markers, crayons, stickers, pencils, pencil sharpener, etc. in it. He has his own box of crayons in it, too.
The 2 year old's "art box" is just a $1 plastic pencil case like they sell at Walmart around Back to School time. It has washable markers and a box of crayons. If he breaks his crayons, he lives with broken crayons.
I also stock up on the 24-color boxes during Back to School sales, when they're 20 cents each. So it's no big deal to pull out a new box if I need to. I think I've only done that once, though, and we have about 10 boxes of crayons in the closet!
Another thought--if your 5 year old usually keeps her crayons in one piece, but breaks a few and ends up with stubs, you could just replace those broken colors with fresh ones from a new box. I bet the 2 year old would be happy to have all the broken pieces from the older kids. mine sure doesn't care!