I would caution against having them in your lives, in any capacity, and leaving it till later to tell your own kids who they are. If you leave it till high school, they will probably resent you for patronising them and they may end up feeling duped.
If you told them sooner (your eldest might appreciate being in the loop at this stage), it would not underestimate them to tell them what the relationship is and that they can continue to call these people by their first names, not the relationship 'title'. You could even explain to them that Daddy's daddy is their Poppy, and this other man, 'x', is a different type of grandfather - keeping the name for the relationship formal, so that when they're a bit older, they can consolidate the formal name with the biological link, not the social/family/important relationship.
It would also explain to them, when older, why these people have been kept more distant.
I just fear a backlash if you hide the biological relationship from them, but not the real life people. Then, when old enough, they can make their own decisions about where they want them to fit in their lives.
ETA: Sammiejane, snap!
I realise it's really hard to be where you're at with this, and maybe you could even leave these truthful explanations on a more 'when asked' basis. If and when they ask would be the time to tell the truth, if you decide not to tell them sooner