C.,
Hello. First off, thanks for being a caring foster parent. That's a really hard job and thanks for being there for children who need it.
What worked for us was moving to regular underwear and a few other tips I have. I let my daughter pick her underwear out at the store and our motto is to keep them "clean and dry", we say this every time we put our underwear on, I tell her she did a good job keeping them "clean and dry" when we go to use the potty (if they are), and remind her again when we pull them up after she uses the potty. Find about 3 days when you can just stay at home or in your neighborhood. I would take each one to the potty - to just try every hour and then explain that he/she won't have to go so often if he/she starts to tell you when he/she has to go pee or poop. I would still be aware of the time; children do get busy playing and don't want to stop. So, if it's been over 2 hours, I'd just say it's time to "try" to pee or poop.
Accidents will happen, and I never made a big deal out of it; however, it was my daughter that had to clean it up - meaning, she had to take her own shorts and underwear off, wipe herself clean and if it got on anything else like the floor, I gave her a wet cloth to clean it up (of course sometimes I would still need to go over it more), and then she had to carry her wet clothes to the washing machine, come back wash her hands and get re-dressed. I would ask, "Where do we put our pee and poop?" She would say, "In Potty" and I said "That's right, let's do that next time." I have always been kind and understanding (even when I just asked if she needed to go potty), and just talked her through each step of clean up. It's just natural consequences. After about two to three days of having to fully clean up after themselves, I think each will start using the potty. They are going through a lot being in foster care, so I would be kind and understanding at all times during the potty training process.
I would move to underwear and have each one clean himself/herself when accidents do occur, but that's just me. We did still use pull-ups for nap and night until those can be mastered too. Sorry this is so long, hope it's helpful. Best of luck to you.
~ J.