[PanelDeland]

I think bobthecop and Classic heartbeat have the plan.The shop should have rules and everyone should know them.Since you are the senior mechanic and probably have a better idea of safety rules etc. you are the obvious choice for setting the rules there.As a way to show everyone they are a part of the family have them help with planning the organization and decor of the parking garage.Do some creative stuff in the parking garage,I saw a guy who had hung the lawn tools to spell out the families last name,put some shelving in and give each of the kids a shelf or two and label it for their personal stuff.
If any of them want to share time in the shop make sure they have a project they are expected to work on and complete.A friends dad taught his brothers to mechanic by dumping a junk engine outside the shop door.If they wanted to hang around the shop they had to work on disassembling that engine.The two of them had torn down 6-8 engines between ages 6-12.Don't hang the bikes on hooks,you will constantly be going to get one down for the smaller kids.Keep things simple and keep a list of chores,dumping the trash,sweeping the floor,dusting the shelves,and such,.Use a rotating schedule so they keep the parking garage and they will feel a sense of pride in keeping that area nice.
Just some suggestions,everyone has to come to their own arrangement.Probably best to sit down and discuss what you are wanting and how best to deal with the situation with the Mrs.so that you and her are on the same page.Hope it works out well.