I have taken in children at one time also. It is life changing experience for them. Even if they knew you before, they are now living under your roof and something disturbing has happened in order for them to need to be there. I repeat what other people have said. The kids will need a lot of time and help adjusting. They may seem fine, but you will be surprised at things that will pop as long as a year after you have had them! Things they never in their lives told anyone else and tried to forget.
There could be many reasons for bed wetting. One little boy would go to the corner of his room every night. He had not been allowed to leave the room at night! My suggestion would be to do two things.
First remind them they are special and you love having them there. Then tell them that because you love them you want to help them learn some skills they will need for life when they are an adult. One of those skills is to learn how to now wet the bed. Tell them you like them to think about it and how 'we' can do that. Then just wait. Do nothing. Let them think about it. In a few days or a week ask them if they have come up with any ideas. The key is to let them be in on it some way.
If you choose to wake them up (what I do) then you could do several things. Set an alarm in their room as well as yours. So they are actively having to participate and it is not really you waking them up (less resentment or struggles that way), then have everyone go potty and get back in bed.
Do a reward system (I would only try this for a short time as we don't want to discourage them and damage them anymore than they already have experienced). You could reward them for waking up with the alarm, for going potty, for having a good attitude about it all, for being dry. Work it however you want. For reward I would do things like have a special breakfast food, buy special underwear (whatever theme they like) since everyone can use another pair of underwear, go on a walk. Notice all these are group activities that include any or all children. Because I realize it will be hard to single out one child and your own children will have their own 'sharing mom' issues.
You could also make sure there are adequate night lights for them. Get up with them a few times at least to make sure shadows, noises, etc. are not part of what keeps them in their bed. ;-)
It will take awhile and frankly I would not even tackle the problem until they have lived with you for several weeks. Think of washing bed pads as nothing different than washing for twin babies who spit up on everything. ;-)
Good luck,
ts