As your DP doesn't speak the language it will be very very hard (sorry don't mean to sound pessimistic) but it is never too early to start. The ideal age to teach a child a second is the day they are born !
My DS has been hearing two languages since he was born, he understands our mother language perfectly and says quite a few words now (he is 25 months), but his English is poor as we don't speak to him in English (this is deliberate as the best thing is to separate the language of the home with the language of the community).
Your best bet is to get your in-laws to speak to your children in Greek CONSISTENTLY, they must't start giving in and speaking to them in English again. According to language experts, young children associate language with individuals, so granie speaks to me in Greek, mummy speaks English, daddy speaks French, etc... Once they have established that you only speak and hear ONE language with that individual it is easier for them to answer back in that language.
Unfortunatley my sister's 3 children do not speak our language despite the fact that we are all from the same country, because she gave up speaking to her kids in our langauge once they had learned English at school. This is where it usually starts going wrong. Lack of consistency.
Your in-laws and relatives will have to be very consistent and patient if they want your childern to learn Greek. Also it is a good idea to get them enrolled in a local Greek school or playgroup in your area if you have one. The more exposure they get to the language the better. There are plenty of bi-lingual nursery rhymes, books, tapes, video's etc. which are also a good start.
In the end they may only become "passive" in their knowledge of Greek (understand it but not speak it), but I reckon even this is better than being monolingual !
Good luck !