Pro Tip: calling developers "resources" causes employee turnover.
What is a "senior" task and what is a "junior" task depends on the organization of labor.
It's certainly possible for a senior to divide a big task into small chunks that juniors can dispatch with great haste. You could possibly put 1 senior and 4 juniors together in a bullpen and get them to do the work that (say) 3 seniors or 10 juniors could do with a different organization.
To realize that you have to get the human relationships right, a theme I keep harping on is that the people involve have to feel the tasks are worthy of them. Many juniors, in a situation like that, might feel like they are being micromanaged.
Another thing you need is for the senior to have enough of a global view of the situation that their seniority really produces value. If you really don't understand where you are going and you'll need to rework everything you aren't getting value out of that kind of situation.
Around my farm we have a large number of "junior resources" available for construction work. Those young people make silly mistakes that cost time and money and that's one reason why they make $18 an hour compared to a more experienced person who makes $40 an hour because they are twice as productive.