There isn't a limit to the number of stations you can receive, as far as I can tell. The problem with picking up CBBC and CBeebies channels is that you ideally need to scan when they are broadcasting, not when BBC3/4 is - the same goes for all other timeshared channels. I've had intermittent problems when scanning and not always receiving all channels so you might need to scan a couple of times until you get the results you want.
In addition (I presume you are using the cable, not the car antenna?) the cable is susceptible to interference so even if your main freeview box can get all the channels, the archos may not unless the cable is kept relatively free from other cables.
Hope this helps and you have a happy new year.