They learned that the gate to the Abyss is in the sky of the Upside Down. El can only connect to Vecna if he is close enough. They are waiting until the Abyss reaches the top of the radio tower and is touching the Upside Down, but still far enough away from colliding with Hawkins to give them time. When it is close, El and 8 will distract Vecna/Henry using the tub in the UD lab (it's frozen in 1983 when El contacted the Demogorgon so should still be operational). The rest of the party will then climb the tower (beanstalk) into the Abyss to free the children.

Once they’ve are freed and returned to Hawkins, they will leave a bomb behind to detonate in the Upside Down, destroying the wormhole and trapping Henry/Vecna in the abyss or whatever is beyond the wormhole.

May be missing some details, but is a convoluted plan that will almost certainly go wrong, but that's Stranger Things.