How thin is too thin for knits?

Hello - I am feeling a little confounded at the moment! I ordered some inexpensive knit fabric online with the intention of using it for the Parachute Polo and School Bus T-shirt. However, when it arrived, it wasn't nearly as thick as I had hoped! One of the fabrics is a striped slub knit - super soft but also somewhat sheer, seems more suited for a scarf or drapey women's top than a child's shirt! And the other one, that I was REALLY looking forward to (cabana stripes! Woo!), feels like it would be better suited to a maxi dress, with the amount of drape and stretch it has going on. It is a rayon jersey, and the slub knit stripe is a poly jersey with some cotton in it. They are both pretty thin. :-/ I guess what I'm wondering is whether I should stash the fabric for later use or try it and see how it turns out. I'm rather irritated that I may have wasted my money on it...