is it possible to do the training if under 7? how will they know
They won't, they trust you to be honest. Last time my boys did it there were some very small 7 year olds...... Corresponding with some very weepy children who just just stood on the stage sobbing..... And a couple who wouldn't stand still and we're swinging their light sabres all over the place while supposed to be standing still.....(safety issues-the things aren't light) so only you know our can anticipate how your child will respond and behave.
When we were there last month, they were asking for proof of age for small ones. I don't think this is standard but maybe they do spot checks and we just happened to be there on a day when they were checking. So just be aware that they may ask.