A Tekin or any other brushless system is going to be a handful for a 5 year old, esp. on LiPo batteries. It will also require you to upgrade the diff on the Mini-T. You can get your LHS to order a Graupner 280 Speed motor for about $20. It has ball bearings and replaceable brushes, runs much faster than the stock motor, and will drop right in and work with the stock electronics and diff, etc. Gear it down one or 2 teeth on the pinion (run a 12 or 13T pinion). That's how mine is set up for my 8 year old daughter. You'll need metal pinions with a set screw though, the stock push on ones won't work. As far as batteries, go to the Promatch Racing website. They have Fusion 1500's for the Mini-T for $12.99 right now.
P.S. There's a mini scale section on these forums. Probably get more help there than here, this is mostly 1/10 electric.