If you have the receipt, then try to take it back.The worst that can happen is that they'll refuse a refund. It is possible that your DD caught it on something and that has caused the damage, in which case the shop are not liable. Even very strong material is damaged if caught on something sharp.
Possibly it was already damaged when you bought it and just didn't notice. You could always take that approach with the shop.
I work in a shop and it entirely depends on the shop.
If you returned it to me with a receipt and said that your daughter had it on for 10 mins whilst in the same room as you sat down having her hair done when you noticed it, I would give you a full refund.
Other shops may give you credit vouchers,, other shops may give you nothing.
A child's dress should be able to withstand being worn for more than 1.5 hrs - especially if the child is supervised (which it sounds like she was). Netting is delicate - but unless the dress came with a warning I still don't think <1.5 hrs usage is reasonable for a £40 dress.
If it were me I'd take it back and see what they say. You loose nothing apart from a bit of time and you might get a refund or credit note.
I took a dress back once in similar circs - it had devoré velvet sleeves, and the first time I put it on, the mesh backing of the devoré laddered really badly. All I'd done was put it on and sit down for an hour (was my playing-in-orchestra dress). They swapped it for another (which also subsequently laddered, but not as badly).
Worth a shot. They can only say no. If nothing else I'd let them know that it's not great material for a child's dress and have a bit of a moan.
If I was still stuck with it, I'd hunt out a good seamstress and get her to replace the netty fabric with something equally pretty but stronger. It might cost a few more £ but most likely not nearly as much as a whole new dress and dd would be able to wear it again. I bet that's possible.
Got my dd something similar material for her prom from Coast, the labels were covered in disclaimers saying they won't accept returns if the fabric gets 'pulled' as it's delicate and inevitable, and warning not to wear any jewellery when trying on/putting on
a 40quid dress should last longer than one wear. Its obviously badly made and/or designed and is not fit for purpose, so you are legally entitled to a refund. Check out the law, bring it back and be polite but very very firm that you are entitled to and expect a full refund.