More ideas from
5 DIY T-Shirt Designs Perfect For Summer | Lovelyish (love this for a kids cover-up and, if cut shorter, cute dance shirt)

Adventures of a Middle Sister: DIY Tie-Dye Swimsuit Cover-up. Plain white oversized T-shirt, tie-dye kit, scissors, needle & thread. Could use a T-shirt that's already colorful and skip the tie-dye step.

INNER GLOW by DSORDER, via Behance

Very fluid style - uses some watercolor-like techniques, but has also done work with bold, clean lines. See her portfolio for her full variety.