Orange, I really liked the responses you got from JoeJoe, Ginger and TBSakaJ9 but just to reinforce what they said, quit fretting over whether you're punishing your W, or rewarding bad behavior and just concentrate on Orange and S4. If you have plans and can't change them then tell her that. If you would rather spend the time with S then tell her you can take care of him. That's it, end of story. What she does with that time should garner this reaction from you ----> WHO CARES.
My ex and I have moved days, swapped weekends and even entire weeks numerous times since D. I really don't give a crap whether she's going to a nun convention or a gang bang with the Dallas Cowboys, that's water off a duck's back. If I can accommodate her request I do it because at some point I'll need the favor back again.