If your DD has any task or activity she can focus upon intensely for an extended period of time, she will be fine on the ACT.

It's not the ACT, but DS took the SAT in 5th grade to qualify for SET. When he came out he said the experience was "very relaxing", which I thought was very funny at the time. DS is a competitive chess player which sometimes requires intense thinking for 8-10 hours a day, and compared to that, he didn't find the 3+ hour SAT test as being demanding.