S.G.
They are a little young to just clean up on their own. My boys didn't really get it until they were in school where it is strictly enforced. The pointers from the Kindergarten teacher
1. Don't let them take out too much, just 1 or 2 containers. This makes it easier for them when it's time to clean up.
2. Designate a clean up song/songs.
3. Make it a game like beat the timer or see who can pick up the most, practice counting at the same time.
4. YOU have to be involved in the clean up process.
The last one is a biggie until they get a little older. Good luck!