With regards to the pet tube, you cannot get that key at this point in the game, and it is not required to move the story forward.  Later on, when you have access to the spells, you can return to this room and get the key.  It is very common in the LEGO games for the areas to have sections you cannot access until you have advanced sufficiently far enough in the game to unlock other characters and / or abilities.

As far as the potion is concerned, I have the PS3 version of the game, and I have noticed that my characters have the annoying habit of backing up a step or two when I walk up to a cauldron, which makes the cauldron menu / button disappear.  The solution I use is to slowly walk towards the cauldron and wait a second or two before pressing the button, to ensure that the character is not going to move.  That solution may or may not work for you on the Wii.