Just don't marry him to Adelaide in the first place. if he, instead, married someone like Gisèle Capet, who is of royal blood and around his age. It's recorded she had at least one son, so it's perfectly acceptable that she's give him 1/2. She'd be 19 on his death, so 2 children is actually very easily done. Let's say 1 girl and 1 boy, most likely named Lothair, Charles or Louis and Matilda, Emma or Adelaide. Let's say their named Lothair (b.985) and Adelaide (b.987).
What happens on their death is hard to say. I'd imagine Hugh Capet would definitely support his grandchildren. His daughter most likely is remarried soon, probably where she married OTL. This let's Enguerrand of Ponthieu still exist, which is a nice butterfly net to have.
King Lothair II of France most likely is married off kinda young, so maybe to Gisela of Bavaria, who wold most likely give him at least one son, maybe 2 like how she did OTL for Stephen I of Hungary. As for his sister, Henry II, Holy Roman Emperor would make a hood match, particularly if she can give him children, unlike his wife OTL.