I would suggest to keep it really simple and be prepared for whatever interested the child, as long as it is safe. Catching fish will keep their attention for a bit, but a frog hopping by or a crayfish in the water is sure to garner some interest and exploring around. I would encourage anything that interests them.
I took my daughters to little creeks; clear, shallow water that they can splash around in a bit, maybe see some aquatic life. Bring a small net and teach them early to respect little creatures and that nature is best left in its place. That's not to say a frog or salamander can't be kept as a pet for a while until it can be safely returned, but its probably best for the frog to just be observed and left where they were found. You can always go back and see them again.
Enjoying the outdoors can grow into many interests and seeing it in person is the best hi-def you can experience.