Hi @Chi37 and welcome to the forums!
This is an odd circumstance and definitely something I would have trouble with. Leaving 5 year old children alone is totally out of order!
First off, let me say that your space looks lovely! I notice, however, that you have it listed as an "Entire Bungalow", which is misleading to guests who would think that the whole house is what they're renting. Your written description is clear that it's an apartment, but I would suggest you change the classification to "Apartment" for property type, and "Entire place" for listing type to better reflect what you're offering. This will prevent future cancellations for this reason.
In your review, it is best to be factual but business-like. Like you, I would struggle with this one. Hoping some other hosts will chime in, as I'm very interested to hear advice on this as well.
I would tend toward saying something like, "Guests did not stay, and asked me to cancel on their behalf rather than cancelling themselves." Star ratings are hard in this instance, however, I would tend toward a 1 for communication, as (although your listing says entire bungalow) it's clear from your description that this is a basement apartment and not an entire home, 1 for house rules, as leaving young kids unattended would certainly be something no hosts would want. Cleanliness: well, I guess a 5, since you don't know. I would then do a thumbs down on recommendation, as I'm sure you wouldn't choose to host these folks again...
What do other hosts suggest?