The original sentence is:
"Mudskippers are fish that use fins to walk on land and even climb trees."
This sentence is grammatically correct. It clearly explains that mudskippers are fish and tells what they do with their fins. The sentence structure and word choice are appropriate for a 7-year-old to understand.
If you want, you could add a little more detail or make it slightly simpler. For example:
"Mudskippers are fish that use their fins to walk on land and sometimes climb trees."