I mean that part is not their fault. It is unfinished. And I do remember him saying something previously about how to fix the gap between the bathroom flooring, and the wood flooring on the landing. So he clearly never bothered to finish it off to prevent them chipping off.
The rest is shocking to be fair. But when you consider that the twins used to be performing monkeys 24/7 for content, such as the staged standing on kitchen cupboard shelves/fridge shelves. It is no wonder they think it is perfectly acceptable to climb a bleeping radiator.