They look like they may take little bits of raw hamburger, ...maybe some local plant matter from wherever the nearest wetlands. If your intention is to let them go, though, sooner might be better. Odds are good they're going to be solely focused on escape for a while, and that won't make them any stronger.
There are a lot of turtle-lovers on the Internet, though. If you find one of their boards you can probably get an idea what the best thing to do is. Odds are they were on your lawn for a reason. The nearest cover to where they were found might do for now.
That Nikonian friend I keep referring to found a turtle out on the road, and we looked for some resources. I don't know the variety these ones might be, (From the shape of the shells they may be females, if I'm remembering properly) ...One thing of interest is that some turtles will only take food which is submerged. I don't know if these are among them. Just in case they actually are hungry, try some of both, above and below the water you give them. I guess some species have their specialized feeding habits and that's that. Sorry for the generalities. But there's people out there as interested in turtles as we are about Pentaxes.