Even though its only t-ball, be prepared to have some parents mad at you. Some think their little Timmy is the next Mike Trout and you are holding him back. Other than that, teach the basics and make it fun for the kids.
Have fun. Get parents to be engaged and volunteer to help out with things. It's especially nice to have several parents helping so you can practice multiple things at the same time. Like station drills. It's hard to stand up and teach skills to 12 tballers. Usually 2 or 3 will be playing in the dirt, a couple will be watching their parents, a couple will be chasing butterflies or grass hoppers and a couple will actually be listening to you. If you break things up into small groups, 3 or 4 kids at a time, you can keep all of them engaged and working. That's the biggest challenge w. that age group. They have zero attention span.
I coach my daughters softball league and it's fun. You have to make it fun for the kids, as long as they are having fun it's all good. Some of the parents can be a handful about their kids you can't let it get to you. At the t-ball level stick to the basics have fun and treats always help.
I did this back in the day. Have fun, stay fun, allow the less athletic kids play more and give these ones extra time and attention. At this place everything is BIG and FUN. Winning and the all stars will have to try to enjoy the ride until little league. It is a good thing you are doing.