Babysitting: watch the kids or play with them?

I just started watching a 2 ½ year old and a 7 year old and I am wondering if I should be trying to play with them when they are playing or should I allow them to free play.I also work in a church nursery and there the rules are to get on the floor with the kids.But I am realizing that in all honesty their parents probably aren't sitting on the floor drawing pictures with them.I am totally okay with playing with them. I just want to know what everyone else's opinion is on whether I should feel guilty for not getting on the floor and playing, as long as I am watching them and making sure they are sharing and having a good time.Edit: I am watching these kids separate from the church. I'm watching them from 8-5 twice a week. The older one is more introvert and shy but really enjoys playing with her sister and the neighbors. The 2 ½ year old is, well a 2 ½ year old and kind of does whatever. via /r/Parenting http://ift.tt/2rMJmdQ