Hopefully this won't sound too complicated-don't want to lose you if you've come this far
........
DD (8 1/2) currently has a 12.2 Dartmoor who she is doing very well with. I'm very aware that, due to her being quite tall for her age, he isn't going to last her for more than 1-2 years. 
My Dpony was used as a stallion by his previous owner and then gelded and I bought him. I've broken him myself and he's been an absolute super star (see riding log for what he's been up to) I've seen on his old owners website a 2 year old filly sired by Dpony, who appears to have his laid back / soft nature and loves attention. She is to mature to 13.2 / 14 hh and is out of a good sized Dartmoor mare.
So, my thoughts are...
We buy her and next year I back her and bring her on over the summer, with DD (then 9 1/2) getting involved whenever possible.
Then the following year DD "takes the reins" if filly is what I hope she will be.
DD would have Datrmoor until filly is ready (2 years time)
Any thoughts appreciated. Is this do-able? Am I dreaming?