I tend to agree with not pushing the scholarship angle her. That could backfire on you in a big way.
If I understand your priorities, you want to create a group of runners that are committed to each other, themselves, and the program. At the most basic level you need to show that you value them and their efforts.
T-shirts are nice, but extrinsic motivators are unlikely to yield the team dynamic you seek. I'm not saying to avoid the t-shirts but don't make it a central part of your off-season.
Create a newsletter, blog, private facebook page, etc. where you can recognize their training efforts each week as well as celebrate things they've done outside of running. Avoid defining them by their times.
Encourage them to use a website for their training logs where they can share their thoughts and feelings about their training with each other. You might need some external motivators to get this process started. If you can still run, include yourself in this training log so they get to know you and your tough spots, too.
There's a lot of truth to the idea that you have to actively seek out those times when their doing the right things and really make a big deal of them. Young athletes are going to regularly fall short of your expectations when you are developing a program from scratch, so make sure they know you've noticed when they're doing the right things.
Have a couple summer runs that are followed up with cookouts where their families are invited. Getting parents on board early will make everything else MUCH easier. Until parents know what you value it's hard for them to fully support your program.
Of course that means you need to have really thought through that yourself. Be crystal clear in your mind what your values are. Write them down. Rewrite them. Many times. You are going to find what you do is more important than what you say, but if the two are aligned your message will get through much better. If your actions and your words conflict, it is going to be hard to get kids or parents to believe in you and your program.
Let them know even if they are not 100% bought into your program yet that you are 100% in their corner.
It takes time, but a program with integrity produces kids that have deep roots and creates a really special community which is what I think you are after.
If you just want numbers, provide ice cream at every summer practice, have multiple overnight camps, and give them lots of free stuff.