couldn't you offer them courses in your dojo if that's logistically possible? Then they should be covered like any other student.Your foundation couldn't possibly reject that, could it?
Our dojo is in vicinity to a state boarding school for children from problematic households (children who have been taken away from violent parents, parents parking their kids there while they do a detoxication cure, children just abandoned by parents who don't care...) and in the last years we always had some students coming from there. They were insured like anyone else, and they weren't more or less violent than the rest of us. None of them stayed until reaching a high grade, but then none of the other kids does, neither (there is some room for improvement in kids' permanence).
I wih you much luck wiht your endeavour!
Teaching Aikido to less priviliged children is an excellent idea. I think Eva Roben's idea is a very good alternative.
I know nothing about the law in Greece, but in the Netherlands it is best that the participants become members of the dojo. Only then are they insured - not if they are doing a course. As for the instructor - he has to be certified by a National Aikido organisation. And then you also have to check if you are personally legally liable in case something happens. In the Netherlands this is not the case with sportive activities, you do that on your own risk. Although in one case an exception was made; in a club a group of gymnasts were practising with experienced assistants, but without a certified instructor. An accident did happen and the club was made responsible.
But the law on this is not the same in each European country, so you should try to find some local legal advice.
Good luck with this initiative!