DL loads the body more than BS because you can generally lift more weight with DLs.
DLs are more effective for gymnastics because it's more in the range in which we move. Same thing with runners/sprinters too... DL more effective for runners than BS.
For the younger guys I would avoid going below say 5-6 RM; there's really no reason to work in the lower RM range when their capacity to build strength is impaired when younger (e.g. before puberty). There's no reason not to work up around 8-12 RM with these guys.
After/during puberty which hits boys and girls around 13-15ish (girls it's usually 12 but can be delayed in gymnastics) I'd start loading them more into the 4-8 RM range. Maybe the occasional 2-3 RM. I don't see a reason to every do 1 RM testing with them except for ego.
But yeah.... DL more effective than BS for the given ROMs that gymnastics uses.... plus when moving up in heavier weights the DLs actually start to help out with gaining strength in the chest/lats for back lever/front lever/etc.