Ah, you've got me wanding off down memory lane! 
...
I was given 20p to go buy what I liked at the school fair when I was little.
I came back with a jar of stick insects! (Price: exactly 20p )
I was absolutely delighted!
Mum, not so much.
Despite my young age, I did have the foresight to ask the people on the stall if they were girls or boys, knowing I didn't want to breed them. "They're all girls" they said to me. This was a cruel trick!
They did not say that the reason they had so many of the bloody things to sell was that they do pathogenesis, meaning they can just reproduce without the need for another gender.
We had them a few months IIRC. I loved them. I was into Enid Blyton at the time and called them all boyish girls names, (Roberta, Georgia, etc).
We looked them up in a book (no internet!) and it said they liked a particular leaf that our neighbours had, so I used to lean over the wall and stell leaves off the bush 
(Surprised my mum sanctioned this now!).
We didn't get as far as mass reproduction though luckily as the cat managed to get into the jar one day and there was stick insect armageddon. For weeks later we were finding half-dead stick insects with 3 legs crawling out of the sofa 
Sorry OP, back to your question! You should have really checked first, although I bet the kid loves them! I would advise calling the mum, and asking if you can offer any advice on looking after them. Also apologise for not thinking it through, and say you hope she's OK with it.
Having said that, assuming she is OK with it, I agree it's an infinitely better present than plastic tat.
Incidentally, what do you do when they start breeding exponentially? I'm curious!