Hey, im new here, im a freind of Cerpin_Taxt, i found a way to enter adult spirit temple as a child in MQ:
1.-Looking at the wall, get near the geyser.
2.-Side jump and then immediatly jump attack.
You should be at the top of the geyser, there you can do:
·Jump to the void, and do a looooooooong fall (that happened to me first XD)
·Turn back, and you should see the entrance hall, jump, and somehow, you will respawn as if you fell in an abyss.
·jump to the other side of the hallway and there, you are in adult spirit temple as a child.
I dont know if this was already discovered, btw.