Test in hospital

You don’t say what test or where. Normally you will get most of the costs back. Did you not get a document afterwards?
Ask your mutuel, they will know your precise situation. We don’t here.

And read the hospitalisation policy too. Not sure how we can help here.

We don't know what your insurance covers, and you haven't even said what the tests are for. How can anyone here answer this question?

The only people who can answer this are the people who sold you the insurance policy, or your mutuelle.

riziv.be (Dutch) and inami.be (French) are the official websites which detail the legal amounts which a mutuelle is obliged to refund for each medical procedure. These amounts do not vary according to how much you have paid; doctors and hospitals are free to charge as much as they like but refunds are fixed.
As a general rule anything charged for a necessary procedure done on a child will be eligible for some level of refund.
However, the test has been done and you have to pay so by far the easiest way is to submit the paperwork to your mutuelle and see what you get back.

If you have a global record for your child, all conventionne appointments such as GP, dentist, hospital specialists are 100% refunded for your child. Without hospitalisation insurance, a conventionne stay in hospital could be almost 100% covered too - I was recently billed 7 euro for overnight stay for child totalling around 1000 euro, paid for thermometer, no point in hospitalisation insurance.

Never buy hospitalisation or ambulatory or dental insurance before doing the maths and consideration of probability of risks.

Ps always ask in advance if any treatment is conventionne and don't forget global record with your GP which is crucial for 100% conventionne treatment refunds.