Something to bare in mind with the Mini Mav is that although it has a small footprint, its still designed for adults, and 'murican adults at that. If it does adjust to your child size then you will have a LOT of leftover straps dangling flapping and getting in the way.
And although the mav is a very good bit of kit, it lacks the refinement of many others, the straps are simply straps, if you load it up heavily they do dig in, and they do have a habit of riding up with a good tug.
I think you will struggle to find reasonably priced water bottle holders, and if you do they will be very bulky to have on your chest. The current fashion are for camelbacks which again is excessive for a 40 min airsoft game.
I like to keep about 1500 bbs in a zipable baggy, rolled and tied with an elastic band in my thigh pocket. If you have bbs jingling in a bottle you may as well carry an air horn and neon sign telling everyone where you are.
For two mags, a chest rig seems excessive personally, even a Mini Mav. I have had a lot of success with two fast mags on my trouser belt at my left hip. You can get them for about £15 delivered from ebay.
Backpacks can be had on ebay for <£20, but quality is hit and miss. you will be looking at spending over £100 for a decent one, but if you look after a cheep Chinese clone it could serve you well.
If i'm honest a belt kit does sound exactly like what you need.
Just my 2c
And yes its the tall, 30rd one you need.