What is the point of no return for the "childhood's end" ending for Bloodborne?

I've already seen spoilers for the game so no worrie there. I am playing through bloodborne and enjoying the DLC, I have consumed 4 third umbilical cords. The doll has told me to go visit the base of the great tree. I have not finished all the DLC however I would prefer not to be too overleveled (if that's even possible for a From Software game) for endgame.

At what point is the point of no return and I'll be forced into NG++? Am I locked into fighting the final boss(es) after I certain or can I muck about (in the DLC, main game, etc) all I want until beating the last one?

Point of no return for this ending is killing Moon Presence. So you can kill Gehrman, then die to Moon Presence and continue exploring world until you are ready for the final battle.

As soon as you enter Gehrman's Garden (The base of the Great Tree) you begin the boss fights.

You aren't necessarily "locked in", however, until you complete the boss fights. If you die at any point during these fights, you will be able to continue exploring the rest of the game at will, and reenter the boss fights at will.

However, if you defeat all of the boss fights, that's game over. The next time you regain control of your character, you will be in NG+ (your next playthrough).