Does adding milk to cereal make it harder for our body to absorb the vitamins and minerals from the cereal?
Overall, no it doesn't. Cereal is made in a way that it's expected consumers will consume it with milk. Here's a list of some important fortifications added to cereal and milk effects on absorption. 

Riboflavin: No. Affected most strongly by light. 

Iron: Yes. Calcium can affect absorption. Milk causing a deficiency is rare unless it is being used as a breast milk substitute in babies.

Thiamin: No. Unless you are drinking alcohol with your cereal.

Folic acid: Milk is a source of this!