Yes — your sentence is correct!
Here is why, step by step:
- Frogs = more than one frog (plural).
- have = the correct verb for a plural subject.
- long back legs = what the frogs have.
- for jumping = tells why the legs are long (what they are used for).
So the grammar is fine. If you want other ways to say it, you can try:
- Frogs have long hind legs for jumping.
- Frogs have long back legs to help them jump.
- Frogs' long back legs help them jump.
All of these mean the same thing and are also correct.