Sorry, I'm not familiar with this specific fanfiction, so it's difficult to recommend similar ones. You could try searching on fanfiction platforms using relevant tags like 'Fenrir', 'Harry' and 'children - related' to find similar stories.
I'm afraid I can't recommend similar fanfictions right off the bat. However, you could look for fanfictions that involve the characters Fenrir and Harry in a more family - oriented or youth - centered context. Check out popular fanfiction websites and browse through the works tagged with these characters and themes like 'protection of the young' or 'youth - focused adventures' which might lead you to something similar.
Since I don't know the exact nature of 'fenrir keeps harry just for children fanfiction', it's tough to recommend similar ones. But generally, if you like stories with these characters, you can start by exploring fanfictions that have a positive relationship between Fenrir and Harry. Look for ones where they are involved in a story that has elements of caring for or being involved with children. Fanfiction.net and Archive of Our Own are great places to start your search. Try using different combinations of search terms related to the characters and the children - related theme to find something that might be similar.
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