If I needed to put DS in care, I would lean towards a home care provider my self. I really believe in multi-age groups (not sure if that's the correct wording) and that is something that cannot be had at a center. older kids learn and enjoy helping young ones, and vice versa.
For the field trips, I suggest if you plan a major outing, do like the schools do, and ask for volunteers. Granted you are there for them b/c they are working parents, but they may be able to take the day, or part of the day off and experience these fun places with their children. And be an extra set of eyes and hands for you. I don't think I'd do any car type field trips with less than 2 adults.
I worked in a military CDC for a couple years, working with pre-schoolers, and we were the only age group that did field trips. I kind of enjoyed them, but mostly wish we never thought of going. Even with a lot of parental help, and plenty of us worker bees there, they were more stressful than enjoyable. Like you, my classroom basically had a loose routine, and we were pretty flexible. I mean, if your 10 preschoolers all decide it is going to be a short nap day, you are pretty well outnumbered. Need to go with the flow!
that is why when people ask if my 4 month old DS is on a schedule, I say, well, it is more of a routine. the times vary. I explained to my dad, who actually GOT IT, I can't realistically expect DS to wait another 1/2 hour to eat when he is hungry (even though he may have JUST eaten, or at least it feels that way). He doesn't understand that. and I can't force him to eat just because I think it is time either. I think this is true for any age group. The only things that happened for sure on time were meals and snack.
I think you have a great thing going, and hopefully it will come to fruition sooner than later!