What type of facility is most important to you? Shower? Bed? Kid friendly zone?
http://www.heathrow-airport-guide.co...acilities.html says there are 3 play areas in T5. I know a 9 month old isn't likely to want a climbing structure, but a place where kids are welcome to crawl/roam around might be nice, and there the people are also parent-type people, so you won't be getting hostile glares because you have a young child in tow.
That page also links to other facilities within the airport, like rooms by the hour for naps, and spas. With a 4 hour layover, the parents might swap off getting a shower or a rest while the other minds the child.