We used Arm and Hammer sensitive briefly but it took forever to rinse and was just not working. He got rashes from it too.
We now use Crunchy Clean because the reviews were great and it's cheap. It works really well! I use it on diapers and regular clothes. I did find that one of their scent mixtures, Apple Toffee I think caused a rash for me. But that's the only scent that has.
Our wash routine is rinse, hot wash on inserts (Flip and Gro baby diapers), rinse. Then for shells rinse, warm wash, rinse.
With arm and hammer it was like that, but with 2 to 3 additional wash cycles with no soap just to get the soap out. I hated it.
I'd suggest that if you use Crunchy Clean use an EO scented one since it's essential oil or go for non scented first. I went with just Tulip scented first and my son got a rash, but it cleared up so I don't think it was the detergent, we still use it.