It's down to you what you think, but you shouldn't voice your opinion.
The two of them must make that decision by themselves.
Your son-in-law does have a point though. Children are time consuming and expensive.
It's not two pairs of shoes, it's three. Three sets of clothes instead of two. Three sets of school trips to pay for and three sets of parents' evenings which may well clash.
Three sets of university fees when the time comes
Can your daughter and son-in-law afford all this? Even if they can, it's not your place to interfere in their decision.
Has anyone seen Mint on BBC 1?


