How to select line weight

I had not been flyfishing in years. Then my kids got me a nine foot nine
weight flyrod that I have been using for surf fishing in the Gulf of Mexico.
I have a nine year old grandson who likes fishing with a spinning rod. I
still have an old flyrod that is probably 40 years old. I would like to
teach him to use it for bass, bluegills etc. in the local ponds.

This rod is not bamboo. It is likely a plastic/fibergas construction. It is
7.5 feet long and has no ID on it at all. How can I determine what weight
line would be appropriate? I imagine we will satrt with poppers or small
streamers.

Charlie