A 50 is great for them to start on. When my daughter started she felt "safe" on that little bike. The are not far off the ground, nore have a tendancy to rip out from under them...but stil giv'er. As for brand ...your choice. Any "branded" bike has good resale in comparison to Chinese. Also be prepared for expensive mods or a new bike in the very near future...
My 11 year old daughter and I have ridden the trails out of town here for the last 3 years, atc and mx and have had the best time. Its great watching her attempt new things on each progressive trip. I think I cried a little on a beautiful fall day watching her 2-wheel drift around a corner for the first time! Its great fun, and now my boy is old/strong enough to really try and RIDE! I encourage them to ride smart, defensive, and technical.
Good Luck, Warrior91