Some infants die before the reach the age of 1. And some kids die before they reach the age of 5. So, will they too be questioned in the grave? And will they too experience the life of Barzakh? And squeezing of grave? If they die as a unbeliever?

They will not be questioned, as there is no meaning for questioning them, and the grave squeezing is not a must for every one. Yes, they will be in Barzakh, because it is a transitional period.