The channel numbers will be the same as usual. CBBC normally has the daytime hours on BBC Three's frequency, so it's that what will effectively take Parliament's daytime hours. Parliament's channel number will just broadcast the standard red "off air" caption used during BBC Three, BBC Four, CBBC and Cbeebies downtime.
CBBC and BBC Three appear to share the same transmission material (cue times when the CBBC DOG
has appeared on BBC Three programming) so I presume there's a spare transmission "system" to run BBC Three all day and CBBC at the same time and feed the DOGs?
Of course this might be testing the waters for an extension of BBC Three's hours to see how it goes down. Looking at the schedule posted above, I think we can say it ain't gonna happen any time soon as even Total Wipeout repeats are now shared between BBC Three and CBBC.