Children's ages vary...newborn (which this obviously won't work for....) to 18...
basic things Drink, Blanket, Bathroom, Pain.
I was thinking about doing it both picture and word based...
Given the age range, I would def include both pic and words on the hospital touch board. I designed an AAC touch board for my Father to use in the hospital & rehab, after he had a stroke--he is aphasic. Including both pics and words is fairly simple to do. I just included the text beneath the pic within the same block on the board.
Along with the general 'needs' (drink, bathroom, etc.), be sure to include more personal touch blocks such as for Mom/Dad, sister/brother... so that the child can ask for their parent, etc.. For my Dad, it was also important to him to be able to communicate things such as saying "Thank you" and "Hello"/"Good-bye" to his family & other visitors (teens may appreciate those blocks as well). And don't forget the most basic "Yes" & "No" blocks, along with at least a few emotions (such as happy/good, scared, tired, etc.).
You can do a quick web search for AAC touch boards, to give you some ideas that can be used as a jumping off point. There are numerous, pre-made hospital communication boards available. I found that none of them really 'fit' my DF's situation very well though, which is why I designed my own. You can make the touch board as simple as you feel is appropriate, while still covering the basics. I think it is important to focus on what sort of things a child would most often want or need to communicate. Some of the pre-made boards tend to include a lot of stuff that really is not practical for the average patient (esp. if that patient is a child).