Hi Charlotte
As the father of two 20-year olds, I'm afraid that I can't offer much practical advice.
Which airline are you flying with? Best to contact them and see what kind of services they offer parents travelling with babies e.g sky cots, heating up pre-prepared food, nappy changing faclities in the loos etc. Might be worth trying to book a seat near the loos and/or by a bulkhead.
Once on Tobago, most of the usual baby requirements are readily available, at reasonable cost, in Penny Savers in Cannan and Carnbee - disposable nappies, jars of baby food and powdered milk etc.
Not sure about the availability of these items in and around Top River Pearl, but you could always enquire from them by email.
Come to think of it - a section in the general advice section, on catering for youngsters would be a good idea - anyone with experience fancy writing a little hints and tips chapter and putting something back into the site for other visitors?
Cheers
Steve
PS You'll also find a list of pharmacies and doctors here -
http://www.mytobago.info/medical.php