Of course Doc feels it vitally, desperately, important to do RIGHT FUCKING NOW to "save" Marty's kids. Yet doesn't feel Marty needs to know, at all, about the accident he's about to in that'll ruin his entire future.
Well, on the other hand, Marty just grows up to be a loser who can't play the guitar and will never end up like Bill and Ted nor go on to have a successful music career. But Marty's kid ends up on the cover of FUCKING USA Today
for trying to rob a bank.
But your point about "right now" is well taken. They've got all the time in the world. 30 years, in fact, to formulate a plan.