You shouldn't have lied to her and given her false hope.
I know little about stick insects, but would doubt that they are very long lived. So, I might well go with Joffrey's suggestion of replace them for now, but when the inevitable happens in the not too distant future do not fudge the issue. Be honest without being brutal.
Finding out what death means is one of the main reasons for keeping animals as pets isn't it?
Err, no, not really the main reason although obviously it comes to all living things eventually.
I have a 15 year old cat, a 12 year old labrador and an 11 year old cocker spaniel. The main purpose for having them is because we wanted them and they are much loved family members. My youngest (DD3) is 14 now and so the pets have been around for as long as she can remember. Likewise my DD2 really, as she is just about to turn 18. They have grown up with them and adore them.
In that time both of their paternal grandparents have sadly died so they are well aware of the cycle of life and death.
I know my pets are elderly, and perhaps on borrowed time, but still healthy and able so we just enjoy them each day. Death will be dealt with when it comes. It won't be fudged over or made a joke of.