Green card expired 4 years ago. Can I renew?

I adopted my boys 14 years ago from Russia and have been under the impression all this time that they were citizens because I've been told that since both their adoptive parents are citizens they became citizens too. Recently we found out that they that they aren't citizens because only one parent (me) went over to get them. They can't become naturalized until they are 18. I never had their green cards renewed because I didn't realize I needed to. Now their green cards have been expired for 4 years, can they be renewed or do they need new ones, if that is even possible.