How to get random order of children editors

I have an concept Widget thas has children of abstract concept Property having an expression node from the base language. Each of concepts inheriting from Property has its own editor looking like x = <expression> in the model editor.

The questions is how to create an editor for the Widget concept so that I could do all of those cases:

widget <name> x = <expression>, y = <expression>
widget <name> y = <expression>, x = <expression>
widget <name> x = <expression>

Also a related question is how to specify which set of Property children are acceptable for the given subclass of the Widget concept. So that for example I can express in my language structure that concept W1 has X sub-concept of Property and concept W2 has X and Y sub-concepts of Property?

Thanks, Jacek