I don't have 14 year old kids, but I do teach high school....!
If you are in the same park, I think they would enjoy the trust and the freedom to go off and ride (and ride, and ride) as well as people-watch (giggle, laugh..). Some possible suggestions:
1. plan to meet for meals - that way they will have 3-5 hours of time together and you can check in with them. That way they don't have to carry lots of money around, either.
2. use 2 way radios - they will just be a click of a button away
3. plan to meet at certain rides between certain times (give 20-30 minute windows for meeting times because one never knows the length of lines)
Those are ideas that immediately pop into my head. And you and your DH enjoy some time together too!