I grew up as a competitive swimmer. At my club, we started dryland training around 10/11 but we were restricted to running, pushups/situps/etc. We were expressly told no weights until 14 because it was not good for younger bodies. If a child is interested in working out there are so many options for strength building that do not include weights. See if you can find an experienced, respectable trainer (not a new hotshot who spent a few hours doing an online course) who might able to provide you with some ideas.

Our gym only lets the kids use cardio equipment at 10 or swim. Way too early for weights