You may bring a lawsuit if you can prove you did not put the critter in the tea.
Contact an attorney in your locale and keep all the materials you have from the fast food place (cup, etc...)
Good luck to you.
I'm sure you were horrified to find a cockroach in your daughter's drink. However, it's a hard thing to sue for. You will need to prove that you were injured/harmed. If you want to take legal action, you should be able to consult with a personal injury attorney for free.