I too have the policy that we must clean up the first mess before going onto the next one…. makes my life and theirs so much more enjoyable. I too have them singing the tidy up song even before they are asked to tidy up …. kids love to be praised for their efforts….. and this way they don't get overwhelmed with the huge job of cleaning up several different toys at once.
I have the policy that the small cars can only be used during nap time when the babies are asleep, or when they are in their seats away from the tiny wheels, the kids understand the importance of making sure they are safely out of the babies reach, my kids were always making sure every car was in the parking garage when it was time for clean up!! Once you set the routine in place they will do it on their own eventually. It might take a few days but be consistent and it will happen for you. Tell them the consequence of not tidying up the cars ex: throwing them out or taking them away for a few days and they will tidy up faster than you think!!! Good luck