I have recently had a similar (not children) issue with a unit we had in Brisbane.
Our building manager reluctantly agreed to a sound test, indicating all previous tests had resulted in success for the hard-floor owning owner and failure for the complainant. The OC had always paid for the tests $1500.
Invariably the tests passed at the required level for when the building was built but failed the more recent, and more strict, levels. And so it was with my case.
Interestingly, up until this point, about two years ago, the approval process required to lay down hard floors never appeared to be applied as, for the past 12 years of my ownership, I had never received notice of any application to do so.
About 18 months ago, as a result of changes to by-laws and the OC/Strata Manager placing more focus on the approvals process with floor-change, notices appear now every month or-so.
Also interestingly, the owner above me was able to provide a specifications sheet appearing to indicate that hard flooring had been part of the original fitout and so was judged under the less stringent requirements. Also, by-laws have been changed again to place responsibility for payment of noise testing onto the losing party in any subsequent dispute.
How does this all relate to the problem here?
1/ You can reasonably ask the above owner/tenant to place carpet/rugs down which will help the situation but if the resident is a tenant and not an owner then you will be going through this process every time there is a turn-over of tenant.
2/ Hind-sight. When purchasing any below-penthouse unit, and prior to final committment, ask to see all original and subsequent strata documentation relating to the unit above PLUS check all/any old sales photos of said unit, which in my case showed the hard flooring had been changed at some point with who-knows-what level of sound proofing and with not strata records to cover it.
How was my situation resolved?
After going through the mentioning, asking, pleading with subsequent tenants/new owners, building manager, strata manager and committee….we sold.
Problem solved.