Billionaire Boy by David Walliams. Ages 7+.
Billionaire Boy (2010) tells the story of Joe Spud, whose father invented Bumfresh — a moist toilet paper — and became the richest man in the country. Joe has his own sports car (he cannot legally drive), a swimming pool, and a room full of every toy ever made. But he has no real friends.
Joe persuades his father to move him to a state school so he can find genuine friendship without his wealth getting in the way. He befriends Bob, an ordinary kind-hearted boy. But Joe's money keeps creating problems — including a catastrophic birthday party and a disastrous attempt to impress people with expensive gifts.
Eventually Joe discovers that money cannot buy loyalty or love. When his father's girlfriend turns out to be a gold-digger and Bob's friendship is tested, Joe must decide what he actually values. The novel ends warmly, with Joe and his father both learning the same lesson.