First of all, two-child policy is the gift for some only child. For some families, maybe the parents are only child and they also can have only child.
Put aside the loneliness of their child, when their child grows up and they grow older, their child marry with an only child girl accidentally, the burden on their child and his wife is unimaginable heavy.
Their child and his wife have to take care of two old couples. Usually, a young couple looking after an old couple is a little difficult if the pressure increasing twice, how can they stand it. But if their parents have two children, they can share the burden of taking care of their parents. It would be much better.