It's common but not ideal. 2 generations of having children at 40 means the 3rd generation will not have grandparents as children/teens, or grandparents in any proper sense – 90 to 100 years old, probably riddled with dementia and incontinence as all of us will be at that age!
Also, much less time to spend with your parents once you really start appreciating them (usually past mid-20s) and your life also starts being less turbulent. Your parents will also be reaching 70 then so you'll be limited in what you can do together.
Also, anyway, the most obvious point is that 50 year olds can be energetic for 50 year olds, but will never be as energetic (both in body and mind) as 20-40 year olds. The child will have a different sort of childhood.
No judgment as that'll probably be my reality if I remain in the UK! I do think there's something wrong societally (financial, institutional structures) if we contradict our basic biology so much. Not that I think having kids at 20 is good, there is a happy medium.
Actually I come from a (first world) country with public/universal housing (like the housing version of the NHS, for all) and so parents can have kids at more biologically favourable ages. Been thinking of moving home for that reason.