We should really make a chart to help !?
Has a rule most people go for a size 10 to 20 kg above their weight. As you get better and bounce more you'll find that you bottom out more. The harder spring will mean that it's hard to bounce but once you've got the technique you be glad of the harder springs.
That not to say buying a pair in your weight size is wrong, I had to and I can still do a lot in them.
The children sizes are fine but as they are meant for children they do have less power than adluts sizes.