Complex sentence: Father goes inside, and initially is not very happy when he finds the toy box on its side with the toys spilling out, on reflection he realises that children will always make a mess and decides not to worry about it.
Split into simpler sentences: Father goes inside, and initially is not very happy when he finds the toy box on its side with the toys spilling out. However, on reflection he realises that children will always make a mess and decides not to worry about it.