since6 (Gramps), why not make a frame for them yourself in one of my frame building classes? I've been teaching them for over 40 years. Yes it is in my self interest to say this but hear me out. While the up right position is not as fussy to get acceptable, a frame built just to your grandkids dimensions (while still allowing them to grow) will make their rides easer (or faster, pick one). Double butted tubing comes in a variety of wall thicknesses and the lighter stuff would work great in this instance. It isn't just about the weight and losing a couple or more pounds, the ride is more lively and comfortable. Some don't understand that distinction. When I align a frame with cheaper tubing (like hi-ten), there is no spring back, it just stays where I pull/push it. That translates to a dead flat ride feeling as well.
Starting when my daughter was 6, I made her little custom frames. I made another when she was 9. She would always ask me why she was so much faster than her friends. She also took great pride in riding something her dad made just for her.
Herbie Helm (the guy that sometimes helps me teach these classes) and I have observed that older guys typically make better students. They aren't concerned if they struggle or make mistakes and have had more life experience working with their hands. In fact it really doesn't matter how naturally talented you are since we will do whatever to take up any slack if necessary so a student finishes with a pro quality frame.
Frame builder's Supply in Portland has mixte lugs but they aren't your only option. I've got a ton of vintage and modern lugs too. As they grow you can supply them with another frame to keep developing a new hobby. And no this isn't your cheapest option.
Doug Fattic
Niles, Michigan