Since you have only had 1 child, writing a formal questionaire might be a bit over the top, but if you have a good relationship with the family, just tell them that you are open to feedback. Maybe mention it as they are leaving one day and ask them to think it over.

I don't do this myself. My personal philosophy is to never ask questions I don't want the answers to. If they like me and my program, they will stay and if they don't they will leave.

If the parents come back to you with something you don't like, what will you do? So, if they say, you charge too much, or you don't do enough crafts, or too much time outside, whatever. If it's something you aren't willing to change, then you are now in a position to deny the request that you asked for.

Sorry, I tend to think of what might go wrong in any situation. If you do ask, let us know how it goes.