My digestive system was lactose intolerant as a child -- could not digest the enzymes in cows milk.
It is not an unusual situation, however. How much sense is it for a human to drink the milk of a cow?
Human female breast milk is the best milk for a growing infant. Past the infant stage however, there is little need to keep up the milk habit into childhood.
The American Dairy Association has been pushing the "children need milk" agenda for years now and for a reason.
Some people will outgrow the lactose intolerance; others, like me, have to have some prescription medication (I was given Prednisone, this was back in the 1970's) to heal the digestive system. Change in diet usually takes place. I wasn't diagnosed with a lactose intolerant system until I was 9 years old as it was rare back then in the 1970's. Symptoms included heavy internal bleeding, severe cramps, stomach aches. I was a mess. Monthly trips to hospitals out of the city as far as 30 miles away.
All I can say is if parents find their child has the symptoms I had, have it diagnosed as early as possible, then make the change in diet.