I would send her provided she perks up a bit before its time to go. DDs stayed at my parents last Friday overnight and when I picked them up the next day found out that DD1 had been up vomitting throughout the night. She was perfectly happy with my mum looking after her, but I felt awful that I wasn't there for her. I'm sure she'll have a great time just her and her grandparents, and likewise your son having mum all to himself.

Missing activities on the weekend is annoying, but if you're anything like me that's a constant state anyway so I figure its pretty unavoidable for themto miss something here and there.