Photography and family are tightly interwoven. A great way to have fun and get some great shots of the kids is for them to play dress-up and for you to take their picture while dressed-up.
First, source the clothes and accessories. These can be from the parents' closet, but thrift stores and prom/special occasion retailer close-outs are great sources for these items.
The kids have a blast putting on and showing their outfits - and that makes great expressions easy to come by.