My 7 year old DD has played tball since she was 4. Her team provides a shirt and hat. They wear shorts or ball pants. Tennis shoes, not cleats are required but she has a pair becasue her "Sissy" does. she also has a bat bag, 2 gloves, 2 bats, a t and her own balls. Again, just like Sissy. Half of the stuff is her sisters hand me downs. She does haver her own helpmet, becasue the coach only has 2 and I am also worried about lice. I find it relatively inexpensive to get ball supplies. My oldest DD (11) has been playing since age 4, so I have a pretty good idea of what a good deal is. If I see a couple of softballs on clearance, I buy them. I've found softballs at flea markets, yard sales etc.
As far as how to teach her tball/softball...just teach the basics. Rule #1 with little ones is to ALWAYS pay attention. They like to draw in the dirt, play in the grass, chase bugs, anything but pay attention to the game. If you can teach her to pay attention, half the battle is won!!
Just work on catching, both grounders and pop ups (gently). The basics of hitting...where to stand in the batters box in relation to the plate, how to hold the bat, up off the shoulders and back, and how to swing level. Timing will come with practice. aalso, work on throwing using the point, step, throw method (it's how our college coach still refers to the process) And finally, how to run bases. 1st, 2nd, 3rd then home. These are the basics that she will need to play softball. If she can throw, hit and run before she starts softball, she'll be ahead of the game.
And as far as injuries go, my 11 yr old DD broke my finger 3 months ago. She can hit well and I was pitching to her. She line drived it to me and I tried to catch it, out of instinct. Then, like an idiot, I pitched to her again 10 minutes later (the swelling had subsided) and she did it again!!

And of course, I tried to catch it again.

I knew better, and I got hurt. SO it can be dangerous, especially in the older age groups. T ball is not so bad but I would reccommend the batting helmet, at least.