THESE FABRICS WILL MELT IF YOU TRY TO DO IRON ON REPAIRS, PLEASE DON'T TRY THAT.
Lycra, Spandex, Cotton Knit, they do NOT stand heat well.
Well, where is the hole? If it's where it shows you probably need a new one. The fabric in leos is usually spandex, cotton, lycra, etc...those are not fabrics that can fixed.
I make costumes for dancers and I can tell you that not much will help here. I suggest you look at the style number and see if you can find an exact match on Ebay or for sale by a company.
I purchased some Under Armor leos for our girl to tumble in and they are luscious but they aren't cheap. If there was a hole in one of them I'd contact the company and ask them if they have any matching fabric for sale so I could make a sew-in patch. If they didn't then I'd let her wear it anyway, if nothing important was showing then I'd get her a sports bra that coordinated enough to blend a bit.
Some things you can do to mask the hole:
Stitch a decoration on top of the hole, like a crochet flower, a felt/flat flower, put a pocket on it, stitch on a bow, something that's cute and keeps it from being noticed. Just disguise it.
One last thing.
Our girl lost her class leo, it was a unique halter that was going to be worn in 2 weeks for recital. I was frantic. I looked online at the company and that had 1 left but it wasn't in her size so I finally found one on Ebay. Just as I was about to click BUY and pay $40, it only cost $31.95 to begin with, and pay something like $14 in shipping and handling my girl found her leo.
I was so very thankful that I didn't have to pay over $50 to replace it. We used that leo for 2 whole years and she can still wear it if she wants to. So glad they get to a size at this age and stay there for a while.
I hope you can figure out what to do. I know some dance teachers will absolutely not allow any alterations or decorations to a class leo and others don't really care. You might ask the teacher first to see what she'll allow.