Children with working parents is one group. Children with developmental problems is another group. There is surely some overlap, but that is far from saying that children with working parents will always have developmental problems. I can point to some people I know and say "they're an only child" based on the way they behave, but that's ignoring the millions of 'only child's who have no behavioral problems or the millions of people from multi-child households who still have difficulty functioning in normal society.

There is certainly more to a child's behavior than whether their parents were working or staying at home. There might be a strong correlation to households with two working parents and households with other developmentally-stunting factors like parental illiteracy or low education levels, low levels of parental involvement, etc, but I'm sure you would have a tough time proving to any level that, with all other factors remaining the same, two working parents cause more damage to a child than having a stay-at-home parent.

For what it's worth (as long as your anecdotal experience means something to you), my brother-in-law is a college administrator and my sister is a first grade teacher and they have two children who don't seem to have any developmental issues. But my sister has kids in her class with a stay-at-home parent who certainly do.