We gave 7 of our 8 kittens away very gradually over the time from 10 - 16 weeks of age. We kept one behind, and that's one of the reasons I don't think Elsa ever really pined for them. But hey, you might be glad if your 8 kids finally moved out! LOL.
They might be a bit upset if taken too early; otherwise they don't really care after the kits are weaned and more/less on their own. In fact, my queens would usually push or swat at them to go away after 4 months old