well I'm no expert
but looking at this comparison   
Nikon D5000 vs D7000 - Our Analysis  the D7000 is only marginally better with noise at high ISOs so I would say what would e the point of changing the body?
I would spend the money on some decent glass with good VR which will enable you to hand hold a bit better when you start getting down to the 1/50's.
if your sitting far enough away that you could really do with a telephoto, then I don't think the flash is all the relevant anyway.  
its a lot of money but I can not speak highly enough of the 70-200mm with VR it would probably surprise you performance wise in the low light...why don't you hire a lens for one of the events and see how it goes if it works just put the dollars away to get some good glass
While this is not the best example this was taken with my 70-200 on my Nikon D80 it was hand held at 1/30sec (could of been 1/50 but pretty sure it was 1/30) and it was night time at a poorly lit speedway track at Alice springs. This picture has also been cropped so as you can imagine it has noise and its a bit soft. however my point is I captured this at quite some distance away, at night and on a D80 which your D5000 no doubt highly out performs and this car was travelling at lightning speed as opposed to your daughters cabriole 

. so its also amazing what a good lens can do.

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