Here are a few ideas:
1) A family portrait - taken by a professional. Alternative, (presuming you are not a professional photographer), a framed photograph that captures the family.
2) A series of books - Walt Disney; Nursery Rhymes; etc.
3) One of the kits that allow parents to make foot prints and hand prints of a baby. The older child could paint the prints.
4) Mugs (coffee cups) with photographs of the children.
