Of course I can see what your concern is here, but on the other hand, if the parents are paying you for the lessons, maybe you should just make the best of it with these 5 who only have keyboards. They'll be learning at least some basic familiarity with musical notation.
You could compose a simple letter to the parents, saying something like, "I am pleased to have your child as a student.
In the interest of maximizing the progress that my students can make, I strongly recommend that you make arrangements for your child to have access to either a standard piano or hammer-action digital instrument, if this is at all possible. A small keyboard makes it very difficult to get the full benefit of piano instruction."
Or something to that effect. However, I wouldn't advise you to say that you 'require' this..........because some of them may not be able to afford anything other than what their child has, and they may be thinking that compromised instruction is better than none at all. And at least those children then will have the satisfaction of being able to play things on their little keyboards, and perhaps some day they themselves will want to move on to a real piano, when they're in a position to do this.
And in the meantime, you're still earning your pay as a teacher!