It is interesting for the children to see the changes of texture in the condensed milk, but if you cook this in a express cooker be very careful with the tin, as the tin can explode if you do not handle it carefully.
You only need a tin of condensed milk and water.
Put the tin into an express cooker (or a normal pan). Cover it with water and cook for 35 minutes on really low heat (or longer if you want a thicker dulce de leche). If you cook this in a normal pan (again on very low heat) you will need to simmer it for 3 hours.
Open the tin and there is the dulce de leche.
You can it this as a spread on bread or with ice-cream.
It is seriously sweet so a small tin goes a long way...