Well, with this last pregnancy I didn't tell my kids anything about preeclampsia until the doc put me on restricted activity. I did find out later that my 8 year old had already overheard me discussing things with my husband though and was a little worried about the baby.
I had lots of extra doctor's appointments due to chronic hypertension, then gestational diabetes and polyhadramnious. Plus "advanced maternal age". I wound up on modified bedrest once my pressures/ protein levels were high. I just explained to them I got sick with my first pregnancy, so they were watching me closely this time. And I was taking it easy just to be safe. And that the worse that would happen is that the baby would be born a little earlier than expected.
Honestly my 6 year old didn't really seem to understand, wasn't worried, just more bored we couldn't get out and do as much fun stuff.
Personally I wouldn't tell them much beyond normal preparing for a sibling. And in your shoes I might look into some summer camps early next year to keep the kiddos occupied during the day for when you are closer to your due date.
I hope this helps, good luck!
Hypothyroid mom to Connor and Claire
(severe pre-e at 38 weeks & "mild" pre-e at 37 weeks)
& baby Annabelle
(chronic HTN & GD, superimposed pre-e @34 weeks, induction @37 weeks)