I've a 7 year old who also can't sit still, at least not at home, she gets very excitable & literally bounces when talking to us about things she's been up to..we used to call her Tigga as a toddler for the same reason.........so much so looking at her sometimes makes me feel sea sick
like you there have been times I've worried, bt she is more than able to sit still & pour hours into a peice of artwork or something, & she does sit still for lessons at School, in fact I get complimented on how well she behaves at School,
Her freinds come around to play, & strangely one of her friends is a similar build to DD, long & lean, almost whippet like, & she is EXACTLY the same, watching them talking to each other has me feeling dizzy, they are on the go so much ........ I've noticed the other girls who are more stocky & solid in build are completely different, which makes me wonder if as with animals, some kids are litterally just bulid for speed, & so have a lot more natural nervous energy to let go of......... DDs friends Mum says the same, & like me she has at times worried...........but as they can control it at times, it seems it IS just normal for some Kids.
So unless its constant, & not sleeping, & the Schools complaining too then its perfectly normal ;)