Personally I would keep them flighted in a cage. Even if they are clipped, I would be waaaay too concerned about the opened doors. Clipped birds, especially ones that have been flying for some time before they were clipped, can still easily get out the door and it only takes a second. I would be more concerned about the kids in this situation. If you are worried about them opening the cage doors anyway, get a lock with a key or something to keep the doors locked and give the foster mom a key so she can change toys etc. around. The only danger in this is if they fly out of the cage while she's changing stuff, but if they are in a quarantine room it shouldn't be a problem.
It takes quite a while, at least from what I've noticed, for them to grow back all their flights. Neelix still has one or two left to molt and it's probably been around a year since he was clipped. Then again, his clip was VERY severe. He might've molted that last one too, I haven't checked. But it took around a year for him to grow them all back. But he could still fly really well and really fast even when he only grew back about half his flights.
If it were me in the situation, I would keep them flighted because they can still get out even if they are clipped, and the chances of them escaping a predator or flying back tend to be higher with a flighted bird.
It is definitely a hard decision here, good luck with whatever you choose.