perhaps you could get one of those wireless doorbells and use it for the purpose you need. keep the button downstairs and secure the bell near a phone upstairs and have the understanding when it rings the kids are to pick up the phone. it is a cheaper fix than an intercom system.
here is something that may work:
http://www.dealtime.com/xPF-Panasonic_KX_TG5433_Phone
I have never used this type with an "Intercom" so not sure if you could use it while on the line already or you have to put the caller on hold.....dunno
Something to look into perhaps. There are many different type and price ranges.