One more thought:
As a nice, yet un-intrusive and non-rude way to keep in touch with the kittens after they are adopted ---
Ask the new owners to send you photographs. Send them via email, snail mail, anything. Just tell them you'd like to have photos of how the kittens are doing, with a little note here and there about them.
This way -- you have proof they are healthy & doing good! And it will drive home to them that you are concerned for the kittens welfare as they grow into adult cats, versus just birthing them and adopting them out.
The lady I adopted Hammie from did this (she was with the Humane Society, I think I mentioned). Anyways I ran into Bibi several times over the year after I adopted Hammie and sent her pictures, notes, etc. And whenever I'd see her in person I'd give her a great big hug and thank her for saving his life & for "letting me" take Hammie home!!! (Hammie's mom was a stray found very pregnant on the side of the road, so Bibi was very attached to Hammie, having witnessed his birth). Bibi carried a little photo album with her and she put Hammie's pic in there with the other kittens she'd fostered & adopted out. I thought that was really sweet.
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