I cut them into good size squares and use them as hankies. The universe brings me more t-shirts than I can manage with so cutting some up is no big deal.
If you don't want to do that then you could cut up the backs an add ties/poppers/velcro to make them into painting aprons?
Make a fake 'under layer' by cutting off the bottom of one and adding it to teh bottom of another one?
Or make them into little cushions or sheets and pillowcases for a dolls bed maybe?
Or just wear them on days when you know you won't be seeing people who judge you by how stained your kids clothes are....