We are so careful at our gym, both when I'm training (trampoline and gym) and coaching. Rules are:
Only JNR/SNR international need worry about weight train (i.e. gymnasium)
No trampolinists under 12 using free weights, specifically above 2kg.
No jumping anywhere whilst wearing weights (floor, double mini, tramp, from height).
Generally for the rest of us, it's 1-2kg ankle weights whilst doing calf raises or leg lifts. We will sometimes use 5kg weights for specific lunges, tricep dips, or a few other things. 2kg, depending on number of reps.
I do remember in gym though using 2kg weights for dorsel lifts etc.
I'd rather have my athletes use resistance/theraband etc and partner work - there are lots of really effective exercises out there. Body weight is effective, and it is important to train the muscles in the way you want to use them (i.e. lots of plyometrics, developing strength through skill repetition).