Are you me?? I have kids of the same age and am off to Mull at the end of the month!
We booked stop-offs through Roomfor5. A place in Cumbria on the way there and Dumfries on the return leg. Not as cheap as I'd have liked but they looked nice. There seemed to be a few places around Penrith and the Lakes.
A cheaper option that we considered was a couple of Travelodge rooms.
We stopped over once at the hotel at Tebay services on the way to Outer Hebs - still quite a long drive the next day. And once we stopped over at a Travelodge (I think) in East Kilbride - west of Glasgow. They both worked and were ok. But the Tebay one was pre-dd so I have very little memory of it. The East Kilbride one I remember because they had a swine flu cluster there at the time! Was quite easy to find and easy to get away from in the morning, they gave us good directions to the Erskine Bridge.