What should I use to supplement the diet of a lactose-intolerant 17-month-old?

Lactose-free milk. Many dairies now make lactose-free milk. A popular brand is lactaid. Lactose-free milk has lactose that has been predigested to simpler, easily absorbed sugars. Lactose-free milk has the same nutritional value of regular milk. It tastes a little sweeter but it works. Breyers makes a lactose-free vanilla ice cream. Mmmmm...
Lactose Intolerance. You can always switch to lactose-free dairy products to ensure a good supply of calcium and vitamin d, otherwise a basic multivitamin with calcium and vitamin d is appropriate.