This article says 1.5% of Japanese males are hermits [1] which shows the desolation that faces the Japanese young. This article [2] outlines the declining marriage rates.
That's not the decline of the nuclear family - it's the lack of a family at all, and it's hard to see that this would have effects on child suicide, when children aren't male hermits. It's not as if hermits become that way because women turn them down. The children haven't reached the age at which they'd worry about it being too late to have a family. Even then, 1.5% isn't a big enough number for people to worry that it's actually their fate. The rate of divorce is higher than 1.5%, as are a huge number of other risks. Becoming a hikkikomori isn't something I suspect most people, never mind children, are realistically worrying about. Even then, hikkis aren't suicidal, they find some contentment in being hermits.