Yes, turtles eat frogs, as well as snails, small and large fish, bugs, etc.
As for poisonous, you should never add ANYTHING to your turtles' pond unless you are 100% positive that they are safe to be eaten. What type of frog were they? Is it possible they escaped?
If one turtle consumed all of the frogs, he or she will likely become very ill in the near future so keep an eye out. If multiple turtles ate them, you may be okay if the toxin/poison was spread out over many turtles. That doesn't mean that you should feed frogs to your turtles, it just may mean that you lucked out. Be more careful next time.
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Baby Boy - January 9th, 2011! (3 months old)
1 RES - 7" long - Umi (3.5 years old)
1 black lab/hound mix - Josie (1.5 year old)
2 cats - Mysti and Molly (6.5 years old)