My family moved around when I was a kid (my dad was in the Air Force). I switched schools after kindergarten (age 5), after 2nd grade (7), after 5th grade (10), and after 6th grade (11). I don't remember having strong feelings about the moves when I was 5 and 7, and although I was upset about moving when I was 10, I was used to it by that point, and adapted easily to my new school.
The last move was the hardest one, because many of the kids at my new school seemed - from my perspective - to have grown into cliques that they would stay in throughout junior high and high school, and it was hard to fit in with anyone.
So if I were in your shoes, I wouldn't worry too much about the 5 year old, and while I'd expect the 8 year old to be upset, I'd also expect him/her to adapt quickly. Obviously you know your kids better than anyone, so you probably have a better idea of how the 8 year old will react.
As for what to say when they ask why, I'd just tell them the truth. Living in a bigger house means more space for everyone, and being free from mortgage payments means there will be more money for other things, as well as the possibility for you and your partner to find jobs you like. All of those reasons make perfect sense.