Hey, I saw your message on the Christian Community feed. I'm on the questions team.
You asked why God would let your baby nephew die. I know you've probably heard all of the rote answers: God knows what he's doing, his ways are higher, everything works for the good of those who love Him. These are very true. However, I know these answers don't always help the hurt go away. In fact, it can make it confusing. In the pain and confusion, know that God is always with you and will always listen. He will never leave you or forsake you.
To answer you question of 'why would God do that?' simply, I don't know. I don't know his plans or purpose for this, but I do know that you can ask him. As trite as it sounds, God always has a plan. He loves you and you family and has an amazing purpose for you.
If this didn't help or you want specific verses, please let me know.