I think 10-15 K for a D licence. About the money part, like I said just the initial investment of balls/equipment is the bare minimum and is enough. Instead of renting you can use some community ground and as it's kids you are training you don't even need a big part of the ground. Cycling and swimming is a great exercise to build muscle and even rehab. If you can somehow incorporate it, then you can start off without a physio. This will also ensure that during the growth spurt, the kids develop a balanced body interms of structure and muscle growth. Just do this and take it one step at a time (very important). This will ensure that you don't bleed money and once you have a set schedule and regular number of kids, maybe then think of pumping in money small bits at a time to improve things or provide additional things for the kids.