My daughter and her boyfriend live together, and he is the sole earner in the home. They each have a kid plus 1 together. H&R Block told them that he didn't make enough to do his return and now the IRS wants birth certificates. I'm a retired accountant and I've never heard of such a thing! They live in a different city than me and their cars broke down otherwise I would have done this for them. This is bugging me.
For people that don't earn very much, the largest single financial event of the year is often getting a large tax refund thanks to the Earned Income Credit and the Child Tax Credit. These refundable credits can result in people getting refunds for thousands more than what they had withheld over the year. This makes fraudulently claiming dependents potentially lucrative. The IRS just wants documentation that supports what is being claimed on the tax return.
A birth certificate is a direct way to prove a child is yours, so it is not an uncommon document to provide.