Would it bring the child back to life? Would it actually prevent the deaths of any other children?
One major problem with statutes that would require someone to notify the police within a certain period of time of discovering something is that they put anyone who discovers something which perhaps should have been discovered earlier in a very bad position. Suppose someone discovers that their child is missing and reckons that a prosecutor would regard the child as having been missing for 25 hours (when the statutory limit is 24). If the person doesn't notify the police but the child shows up anyway, all is good. If they do notify the police, however, then they would risk being prosecuted for having failed to do so in a timely manner, even if the child shows up unharmed.
To be sure, courts have let the government get away with a lot of Fifth Amendment violations, by limiting people's Fifth-Amendment claims to situations in which the claimants explicitly know the statements will be incriminating. A better policy would be to provide that delay in notifying the police could only be regarded as a potential criminal or civil offense in cases where it can be shown that either (1.1) harm befell the child which the police discovered, would have discovered, or should have discovered, without the person who should have notified the police having done so, and (1.2) such harm was likely a foreseeable consequence of the failure to notify the police; or else (2.1) the failure to notify the police was motivated by a desire to conceal other criminal activity, and (2.2) it demonstrated a wanton disregard for the safety of the child.
Note that in the scenario (1), the sooner the parent tells the police, the better; if notifying the police at some particular time helps them recover the child unharmed, it won't matter whether the police "should" have bee notified earlier. In scenario (2), charges could be filed even if the child ends up unharmed, but only if the caregiver's conduct rises to the level of wanton disregard for the child's safety. If a caregiver who discovered a child was missing were to e.g. spend a few hours disposing of a pot-growing operation before calling the police, even if the person should expect that delay might possibly cause the child to come to harm, such behavior would not constitute a level of wanton disregard sufficient to justify charges. On the other hand, if the person declined to call police and had no clear plan to ever call them, that would likely constitute reckless disregard.
PS—If I had my druthers, such legislation would also explicitly make clear that a person who reports a child missing does not thereby waive any Fourth-Amendment rights; a missing-child report, in and of itself, shall not constitute probable cause for a search of the caregiver’s property. If the caregiver refuses to allow a search that would have given the police evidence that would have been useful toward finding the child, the refusal to allow the search may regarded in much the same way as a failure to report the missing child. On the other hand, if the caregiver believes that a search of part of his/her property would not yield anything that would help find the child, the caregiver has the right to refuse.