main Serve card and then I can transfer that cash to/from my teens' sub-accounts linked to their cards. They then use their Serve card when they go on school trips out of town, buy gas, etc. I keep full control over the funds going in and out of their sub-accounts.
They will sometimes text me, "Hey dad, can I get x-dollars to spend on x-event?" I can easily make the funds available with my Serve phone app if I choose to do so.