Try walking uptown on Madison to the little restaurant row between 92-93rd Sts...on the same block (both sides) as Sarabeth's I much prefer Le Paris and Pascalou. Both are charming French bistros with very reasonable lunch specials. I am not at all a fan of Serafina, kind of a Gossip Girl place.
1308 Madison Ave, New York, NY 10128
1312 Madison Avenue, New York, NY 10128
A few to look into:
Land Northeast Thai (lunch available only on weekends): http://landthaikitchen.com/eastside/
Sarahbeth's East: http://www.sarabethseast.com/menus.html
Cascabel Taqueria: http://www.nyctacos.com/menu.html (lunch on weekdays, brunch on weekends)
Shake Shack UES: http://www.shakeshack.com/
1295 Madison Avenue, New York, NY 10128
Land Northeast Thai
1565 2nd Ave, New York, NY 10028
1538 2nd Ave, New York, NY 10028
154 East 86th Street, New York, NY 10028
It would be helpful to know what kind of food you are looking for, any avoids, and what you consider inexpensive, i.e., per person budget for food only. That said....
You might consider Untitled, the restaurant at the Whitney Museum, on Madison, b/t 74th & 75th Sts. The menu is on the website, so you can decide if it appeals to you and your daughters and if prices fit your budget.
945 Madison Ave, New York, NY 10021