This may come off as sunding harsh but its the truth. The reason why most people can’t handle (or have a very difficult time) losing a young child or baby is partially because they view their children as my child, my baby… its NOT your child or your baby. Hashem chose to make you a Shliach as a custodian over this neshama, its your obligation as a parent to do your best for the child, but its Hashems neshama and He gave it to you He can take it away.
If a child dies at such a young age it is likely that the neshama only needed a small tikkun and whatever pain it went through was that tikkun and it can now be in Gan Eden completely cleansed.
The worst thing a person can do is question Hashem. If you ask Him enough questions He will bring you up there to show you the answers. But the real answer is that there are no questions. Hashem knows what He is doing. Trust in Hashem and He will heal all your pains.