A thought just occured to me

Wanda could have re-created the children at her will, there is no need to jump through into another universe, but she did not.

So now I am pretty certain these children were not created to be that age, I think Wanda in her altered reality in WandaVision, got married with Vision, and gave birth to those 2 children and watched them growing up (bear in mind I did not watch WandaVision).