Hi,
Nicolas might not have understood your question, his answer is if you want users to be able to select multiple values of a filter (like for example select both size 50 and 56).
What you want is not possible by default, a filter can only be applied on a single characteristic.
However, you do not necessarily need to have two different characteristics maat kids and maat for the sizes. You can put all the sizes, both 50 - 80 and M - XXL, in the same characteristic. If you turn on the dynamic display of your filter, it will then only display the available sizes in combination with the other filters. So if "jongens" is selected in the first filter it will display 50 through 80, and similarly if "dames" is selected it will only display M - XXL.
Hope that helps.