When making custom items how do you balance effects that trigger on a natural 20? Is it worth it for items to trigger smaller effects than literal decapitation?

I'm considering giving my daughter's cleric a +1 mace that effectively casts mass healing word on a crit. She'll probably be in the 8-9 level range. Is this reasonably balanced, busted or anaemic?

That depends, does the mass healing word heal the target, the cleric, or an ally?

As long as it doesn't heal the target, for that level range, I think it's balanced :) 

That actually sounds like a cool magic item, I am going to potential use it now 

also make sure to make sure the he party members are also getting cool magic items so don’t feel left out 

Sweet, I'm glad I pitched that about right.

So, the Healer's Knell Mace. It's made of magically hardened rose gold with a handle of rose quartz... because it's been made by the noble family of a little girl's cleric.

The game is my 6 year old daughter and her 3 year old brother so I don't have to be quite as worried about loot envy. I'll make sure the boy gets something neat at around the same time but the other characters are a bear and a warforged who are mostly run by myself.

Oh well I can now see it’s not much of an issue lol, but you’re raising these kids right then 

honestly tho you’re doing a good thing for there kids and I hope they can enjoy dnd for years to come