Teaching older kids how to cycle. Tips and advice?

My two kids (8, 12) have never learned how to ride a bike. Part of this is my fault: I taught myself how ride when I was 15 after my cousin gave me and my sister her bike. I've never been into sports and riding when there's other people around stresses me out. Hence, I barely rode that bike after learning.

Even though my daughter is already 12, I'd like to change that. I took them to the park today and we rented a couple of bikes. IMO, they were terrible: heavy, no brakes, and my daughter couldn't find a good fit. We tried, but it was a mess.

I'm planning on getting a pair of bikes, but I don't know what would work best for them. I'm afraid most 24" and 26" bikes are geared towards experienced riders. Any recommendations that won't break the bank?

Also, any tips to teach them how to ride? Do we skip training wheels at this point? (I'm not even sure there are bikes with training wheels for this age). We tried to work on balancing first, but that was it.

Thank you!