The Hilton in Buenos Aires is the hotel in Buenos Aires I stay at the most. It's located perfectly for me especially as cabs around Buenos Aires -- including from the safe, comfortable area of Puerto Madero (where Hilton is located) to all the main locations -- are a very cheap cab ride away at most. You could walk also, but with young children it may not be recommended if they aren't fans of long walks to begin with. Just make sure to request a room with the appropriate amount of bedding for your family in advance. The only real negative about the Hilton location is that the location may seem a bit too "sanitized" and boring from a kid's perspective; other than that it's enjoyable and some children like to be able to walk along the docks and look at the naval/port police.

What are things your children have liked to do on some other of your trips? El Parque de la Costa, a theme park, and the city's zoo are a good option for children in Buenos Aires. Chasing penguins in the south of Argentina (if you are planning to go beyond the immediate area of Buenos Aires) can be fun too, and going down near Tierra del Fuego to smell the stinking seals always is something kids remember as well. I don't know if it corresponds to your trip dates off the top of my head, but there's La Feria de los Chicos (the Fair of the Children) in the Palermo Viejo neighborhood that happened in April too. They may like that as well.

Edit: the Children's Fair is in May this year, so nix that.