Some acrylics don't hold up as well in shaping as others. And as Knitgal said it won't breathe like a natural fiber. If you want to make your dd a spring/summer sweater, consider making it in wool, cotton or a linen blend (I personally think linen by itself is too coarse for clothing, but love the blends) Take the sweater up a needle or two size to make it more "holey" or find a nice open lace pattern that would be less weight than cables or stockinette stitch alone.
I really enjoy Wool-ease for children's sweaters as they are washable and hold up well. I found one of my dd's sweaters that I knit from Wool-ease something like 12 years ago that has been through three kids and it still looks great. The baby sweater my grandmother crocheted out of RHSS from that same era aren't as nice looking.
"The world now has the means to end extreme poverty, we pray we will have the will"
knitncook there, too!
OTN: Laughing Carrots; Jaywaler
FO: Potamus socks; Palindrome