Doing this can throw off our computations for weight and balance as children are counted as weighing less than adults. Thus why we ask if the passenger is a child. If a flight goes on a weight restriction we look at how many children are showing in the system and if there aren't enough to give us the extra weight space we need then the flight gets a cap and we have to ask for vols and may even have to invol someone. Having all the correct information really does help.