I don't think the whole hotel will be CBeebies themed. It would be narrowing the target market of the accommodation far too much in my opinion which, although there is other accommodation for older guests, is something I can't see Merlin wanting to do. In the plans submitted for the development it does state:
The external appearance will maintain the ‘English Colonial’ theme of the current Alton Towers Hotel.
Whilst of course this doesn't exclude a different theme for the inside, I would be very surprised to see it fully CBeebies themed. I think Lewis' suggestion that we could see different themes in different areas could be a good possibility, and I would have though a CBeebies themed floor would work well as part of this.
Another thing to consider is the location of the extension. It sits next to the Spa, which is somewhere people go for a peaceful and tranquil atmosphere. Anything themed to CBeebies will naturally generate a great deal of noise from the higher energy entertainment and associated noise from excitable children, something which would be to the detriment of the peaceful and tranquil atmosphere of the Spa. Although the Spa operates during the day, the nature of CBeebies' target audience of young families means that they will expect entertainment earlier in the day - most likely early afternoon - which will mean noise could carry over to the Spa. Although the changing area for the stage in the hotel extension does appear to create a limited buffer between the Spa and new hotel, I'm still not convinced the noise wouldn't have a negative impact on the Spa.