Are the child records master-detailed? If so, use rollup to count them. Otherwise you can use DLRS to do the same thing. 
https://github.com/afawcett/declarative-lookup-rollup-summaries

Sometimes the simplest answer is the best. Actually most times I guess. That worked. Thanks Pat!

This is a perfect use case for a “Result of Subquery” Condition. Result of Subquery Conditions are ideal for finding parent records with/without child / related records, optionally filtered by “Subconditions”. For instance if you wanted to find Account records with child Contacts, you would create a “Result of Subquery Condition” on your Contacts Model like this:

Field: Account Id
Operator: in
Content: Result of Subquery
 - Join Object: Contact
 - Join Field: AccountId

This will show just Accounts that have Contacts.

AHA! I knew there was a way to do it. Thanks Zach! That worked perfect too. 

Ok, so how do the subconditions work? I’m trying to do Completed__c != Yes, but I keep getting errors. 

If I do Completed__c !=‘Yes’ it outputs as Completed__c !=‘Yes’ and says is not a viable character. 

If I put Completed__c !=Yes it wants a colon, then if I add the colon it says Yes it not a variable.

If I put Completed__c !=“Yes” it renders the same but says that " is not a viable character. 

Any ideas?

What kind of field is “Completed__c” ?

Nevermind, I was messing things up - same problem as this: https://community.skuid.com/t/subquery-error-using-like-condition

I was trying to put my subcondition in the logic field. 

Ah, that makes sense. Apparently this is a common source of confusion…