My best advice would be to see if you can have them "test ride" both the 50's and 80's-90's. I bought my 6 year old daughter a Suzuki Quadmaster LT50 for Xmas (she turned 7 a few days after Xmas). If you can find a place that rents quads take them there and have them try both sizes. My daughter wanted the 90 becaues it was bigger and at first insisted that she would not ride anything smaller. I was more comfortable with her on a 50 for safety's sake. She had a go kart for about a year and understood the concepts of acceleration and braking and not steering into rocks and trees.

There was place that rented quads by the hour so off we went. She's big for her age, standing about 4'6" tall and about 80 pounds, so physically she was big enough for a 90., but she was not ready emotionally. She got on the 90 first, and when the engine fired up wanted OFF! She then got on the 50 and handled that very well, nervous at first, but got the hang of it. I forced her to get back on the 90 and try it. She handled it well, but after about 10 minutes wanted off and back on the 50.

Based on her performance, I would have bought her a 90 (there's a throttle limiter on both sizes), but she was too afraid of the 90, changed her mind and wanted the 50. I bought the 50 and I'm glad I did. Since she's big, and a now a daredevil, I don't worry as much of her tipping the quad. With the 50 she's lower to the ground and can't go very fast. (When we ride together I can't get out of 3rd gear else I leave her behind.) I'll probably buy her the 90 for Xmas this year or next year at the latest. Buying the 50 and maybe selling it in a year or two was worth the extra money. She was comfortable and I don't worry as much.

So my advice is, buy what you and your daughters are most comfortable with based on safety, not money. It's worth buying another quad in one to three years if your child is safe. Besides you can always sell the smaller quad.