They are Will’s step-children. He brought them up. Helovesthem, as do the other Grundys. I know it’s just a story but he must be devastated to lose them too. It will be a continual ache in the narrative if they disappear completely
They are also Andrew's children. Regardless of what Will thinks or feels there is very little affection now for him from Mia and Jake.
Don’t know how it would work out in rl but doesn’t Poppy have some right not to be suddenly wrenched from her step-siblings?
Aren't they her half- siblings? Poppy is the only Grundy they are related to. I don't know what "right" you think Poppy has but if Mia and Jake don't want to see the Grundys that's up to them.