How can I get out of not goin somewhere?

Hi I need to tell a little porkie to my mate as I said I would take my kids to go swimmin 2morro afternoon but my son would rather go to school in the afternoon as it would mean him skipping the season at nursery also were goin away sat and got so much to sort out my husband is stuck on the motorway right now and she knows this and we need our car to go away in on sat so thought I could say there was summit wrong with the car we needed to go get sorted in the afternoon something little so that we could pretend to take car to garage in mornin and say we gotta take it back in afternoon for it to be fixed but what can I say is wrong with it