Do craft time while the babies are napping and if they don't nap then have them do snack while others craft. A few cheerios on the tray can keep them going for awhile.

I found some big cloth books at a craft fair one time. I put those on the trays and used a strip of fabric down the middle of the book to tie them around the trays. They could turn the pages but not move them. The same can be done with platic books with the covers taped to the tray. There used to be toys that would suction to high chair trays and those worked good too.