Sorry for not being help you with this. But i had problems already at the beginning:
I talked to the witch. But after this i cannot find the mother nowhere. Where did you find her?

She's in the tavern in your capital, in the upper left corner next to the bar. Or she should be. Some people on Steam were saying she never showed up so that might be glitched, IDK.

Yes. she is there. She was not there some time ago when i tried to find her in the whole city. But somehow she appeared then. I have no clue, why.

To your original question: the problem in general is, that there are some game breaking bugs inside the game, which makes it unplayable at some point. We will have to wait, until the devs have released some better patches. Can be, that the old save games are broken then. And when starting the game next time i would go there a bit later, when you have the glitterdust spell, either by learning or by buying a scroll.

Yeah from what I've found if you talk to the spirit inside the boy without passing the Arcana check while having the Glitterdust spell or a Glitterdust scroll you pretty much can't finish the quest. Definitely either a bug or an oversight.

I was able to buy a glitterdust spell from the Cleric girl in the capital next to the stall that sells weapons and armour. I then randomly encountered some lizard men bypassing the need to talk to the witch so I can't tell anyone ho wo to do it that way. At the lizard camp I spoke to the chief, he told me the boy was in a hut in the camp, I then went around the camp and completed 3 different small quests (just small speech checks, needed to keep boy alive). I went to the hut the kid was on, passed the Arcane skill check and instantly used glitterdust (if you haven't learned the spell but bought it, the spell needs to be equipped in a quickslot by a character that can cast it) cast heal and protect evil on the boy and killed the wisp. Sorry if these are spoilers, I don't use forums often and have no idea how to use spoiler tags.

Huh, didn't know you can save the kid by doing that.

You can also offer to trade the Rattler to him. He'll come in and glitterdust the wisp for you, and if you hit the kid with Prot Lightning before you leave the spirit hut to get the rattler he won't get killed. >.>