Peggy,

For the 18mo who is thin for height but lactose/soy intolerant:

- Flax seed, ground up and cooked with the morning oatmeal, will
increase fat content, and add beneficial omega oils.  Flax seed oil
can be added to foods (but has a strong taste so would have to be
heavily disguised)

- There is lactose-reduced milk available

- some people who can't take milk can tolerate yogurt (use full fat
varieties).

- avocado is a nutritious source of fat

- nut butters, such as tahini, almond or cashew (if there are no nut
allergies); peanut butter is good and fattening, but there's a lot of
controversy about carcinogens and allergies right now.

- my two year old loves dipping her bread in olive oil.

Hope that helps,

Jennifer Landels