I am stuck with the preparation of the Grasses decoctions and cannot move forward to undo the spell of the malformed child. Any advice or tips ??

what exactly are you stuck on? when you're given the quest to make them, you need to use your 'senses' to find some of the ingredients on a table, all others should be available in or around kaer morhen. check in the alchemy tab if your unsure. (the recipe should appear when the quest is given to make the potions)