I think it's great, but I really wouldn't put anything about negative behaviour. No point. I would take out that entire section where it says,

"but I have noticed lately that they are getting upset when the other children leave for the day, and they must stay here longer. I think maybe finding a daycare closer to home might be a better option for them, so they are not in care for more time than they need to be. I am honoured that you like my daycare enough to keep them here after you have moved, but this may be the best for everyone involved.

I wouldn't suggest anything to them about their future plans, or what impact the extra hours are having on their boys. Just keep it simple. Do compliment the boys, but that's as far as I would take it.

Good luck!