The ex editor in me finds the text accompanying the question unclear. I think (which I only worked out after seeing the second screen shot) you trying to explain that as people book the number of available spaces will automatically update? It took a few read throughs to get that. You could try:
Are there limited number of places on this trip?
Yes / No
If there are limited places on the trip, as parents book via confirmation of attendance (free trip) card or PayPoint the number of available places will update automatically.
Payments made by cash and cheque must be recorded by immediately to ensure availability is updated. If this is not done the trip may be over-booked.
Removing a confirmed person will automatically update the number of available places.
Or I could have got the gist of it completely wrong. Or I could be jumping the gun and you might not be up to text yet - if so sorry!