It likely costs them a whole lot more to hire multiple people. There are more costs associated with employees than just wage. Thats why turnover costs money. And a few dollars more per hour over three years over how many people in their 20 locations?
This is likely a wash for them compared to hiring more people or paying more to get older people.
Let's bear in mind this is by far the smallest fine they've been hit with for child labor and they keep doing it. I can't imagine their accountants are that bad.