I have 2 10 month olds and both eat bark. Does anyone know why? Is there something that they lack in their diet?

I doubt it's anything missing in their diet - horses will pretty much eat anything within their reach that is appealing in any way.  Make sure they are getting  enough roughage (i.e. hay) and maybe even give them some toys to alleviate any boredom.  If your horses continue to eat bark, you can fence around the trees to prevent further damage to the trees and the horses teeth.