Most of those children wouldn't be able to communicate with their own parents, if that were the case.
Somalis are usually not spread out among the natives. They tend to clump together in ethnic enclaves where it is very easy to learn Somali (and where life is unpleasant for the unfortunate child who doesn't learn it).
And some parents do learn Swedish so their children might not know their parents native language. Or have you inspected every Somalian family in Sweden? It might not be that many, but doesn't make his point any less true.