I will teach you a very important lesson in life: you cannot motivate other people. From inside, inspiration arrives.
So you can use inspiration for fear in general terms. That's the kind of encouragement that tells me I'll shoot you if you're late one more time. Incentive incentive can be used. That's the kind that tells you I'll give you a bonus if you do well. Both work to a limited degree, but particularly in your case, you won't get great results. In order to punish or motivate others, you have nothing that you can do. People just need to want to do things for motivation for development, to develop themselves.

Another useful piece of advice I'll give you: some do and some don't and that's just the way people are.

There's always going to be a kid who doesn't want to participate, like you said. There will always be a kid who loves it. That's about the way that it is. Discover what your tasks are. Often you talk to a kid and say, "it's ok to just sit there but everyone else will be over on the court playing and having fun. If you'd like to come join us you can," and then just let the kid sit there. They'll also join eventually. As long as that is what you can get away with in Upward Games.

It's my personal belief that you should be there for everyone but focus on the people that want your help. You shouldn't spend your time on the people that don't want your help while ignoring the people that do.

One last piece of advice: Be the change you want to see in the world. I think that's a Gandhi quote.