Hi and welcome to the forum
Do you have any pictures of the fish (dead or alive)?
Can you post a picture of the entire aquarium?
Can you post a picture of the test strip next to the colour chart so we can see the actual colours/ numbers?
If you get water tested by a pet shop, write the results down in numbers at the time they do the test. If the shop says the water is fine, ask them for the numbers.
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
Do you have buckets and hoses specifically for the fish tank?
What sort of filter is on the tank?
How often and how do you clean the filter?
If most or all of the fish die in a short space of time, it's usually poisoning. This can be from a new filter that hasn't developed the beneficial bacteria to keep ammonia and nitrite at 0ppm, or poison from an external source (cream, oil, perfume, hand sanitiser, moisturiser, perfume from soap, cigarette smoke, etc). If the water has 0ppm of ammonia and 0ppm nitrite, then it's a poison getting into the aquarium.
Did anyone do anything to the tank on the day before the fish died or in the morning when they died?
Does anyone in the house use perfumes, deodorants, smoke, paint or do anything that releases fumes in the room or near the fish tank?
Do you wash your hands with soap before working in the tank?
If yes, does the soap have a perfume in it?
I'm assuming the aquarium is in your child's room. Do they put anything in the tank (fingers, toys, anything)?