I'm playing Ye Olde Magic Show quest, A Delightful Evening of Mystery approach. The heroes are the wizard and the bard. Previously, my bard's had a child. One of the steps to the quest was for the bard to purchase Premium Lute Strings from the Village Shoppe.

Having noticed an option to have the child go shopping, I've had the bard tell her child to go and pick up the lute strings. However, that object now seems to be stuck in the child's inventory. I open the inventory using the interaction on the child, but I can't move the strings anywhere (I tried both bard's inventory and somewhere on the home lot).

Have I made a huge mistake by having the child go and do the shopping? Is there a way for me to clear this step without having to quit the quest?

No idea what to do with the kid, but you should be able to check the village shop the next day and buy it with your hero sim this time. Quest items that you need to purchase the same way as regular merchandise should keep restocking as long as the quest in active.

To try and "skip" this duty, all you can do is hold your mouse pointer over the duty, and left click on your mouse while at the same time press the Ctrl key. Doing this ends the duty as if you have completed it. However, it may fail in that your quest may fail to end. That can happen, but it also may not. The only way to tell is by playing out the rest of the quest and see if it ends.
Most of the time doing this works in whatever quest I am playing but I have an error in which I cannot return to a logical stage in which to make the error non-existent.
There is but one quest in which I know for a fact this trick does not work in trying to skip a wizards duty to enact some weird spell or ability, in which is not mentioned in any quest in the game. In this quest, it calls for the monarch and the wizard to play together, and to go after a demon I believe it is. I think its towards the war taking place. This quest is glitched, and the wizard is unable to perform the duty, being as it is not listed in self-action options, meaning selecting said sim to perform an action on or by selected sim. So since that spell or strange action for the wizard does not exist, the duty will show as having completed successfully, but the quest will not end, so in truth skipping the duty did not help, but I figure since the quest is glitched, likely there was no way to have completed it anyway.
I am certain you no longer need this help, and sorry, I am late to the rescue, but hopefully this trick is major helpful to any in the future.

Xeny

PS: The name of the quest I mentioned above is "Invasion" (thanks Eggyslav). It can be played with either the wizard, or knight, with the monarch.