Yes. Currently you will need to register each child even if they are in the same school. i.e. if you have three children all at schools who have squid as the online payment option you will have three letters informing you of three different registration numbers and you must set up all three squid accounts. (SEE ALSO QUESTION BELOW ABOUT PAYING WITH SQUID). Squid are currently working on a program to change this but it is not available yet.