Your post isn't clear, but if they are taking a UK phone with them then it will cost you the same as if they were here.
If they are getting a Kenyan phone you could either send the texts from a website/something like Skype. Or if you operator provides it, you can change your tariff (bolt-on or service) for cheaper calls.
Sorry for not being clear. Yes, they would be getting a local sim card. Would 02's international caller bolt on, which i found quickly, be the best then? This gives you 10p texts to anyone in the world for a one off charge of £4.99