The starting nap earlier is one thing to consider. Sometimes we leave kids till they are showing signs of tiredness but then are too overtired to deal with going to sleep. By making it a consistent time regardless they come to see it as a normal part of the day. Put into the playpen awake they have time to orient themselves to where they are and no amount of tears brings you back and they learn to rest/sleep/play till you come. No need for toys as there is plenty to see and blankets and toes to play with.

You might consider a video monitoring system to watch what is happening instead of going in and it might also give you clues as to the process the child is going through. One of the tricks to living through the tears is to set yourself a list of things you must do before you can have the kids up and then stick to it. It helps to tune out the tears. Keep reminding yourself that you are really doing the children a favour by insisting on naptime.

If being in a separate room for napping isn't working maybe you need to start by playpen training them. That means having the playpens in teh playroom and for periods of time you put them in them and then go about doing something. They need to get the idea that you go and come back and they can't get to you.

If you want to go with the sitting with them and cosleeping what about getting a play yard - those ones where the sections go together to make a hexagon. Then pad the bottom with a thick pad/quilt. Put them in there and sit between them while they lay down and fall asleep. Then when asleep leave the area. The play yard isnt' as confining as a playpen. It also works great for a child that is almost ready to give up morning nap as in they go in there for "naptime" and are free to play or sleep.