The best thing you can do for a 12 year old is get them running and develop interest. Finding a good cross country program can help a lot in that regard, particularly when it allows a kid to run with others their own age, but can also allow you to see the girl's potential in longer races than 400m. In terms of actual workload, I'd suggest no more than 4 days with some decent quality each day (similar to what ecuxctf describes).
