If my brother died without a Will but I was named as beneficiary of his accounts and not his children, who gets that money?

The beneficiary listed on the accounts trump a will, or no will.  As long as you brother was of sound mind when he created the accounts, they will go to you.

Legally, you get them and they overrule anything in the will.  However, what you need to do morally is another story.

Insurance proceeds, some bank accounts, and some retirement accounts pass outside of probate and are not controlled by a will.  Therefore, the beneficiary listed by your brother is the person who takes the assets.  The assuption is this is what he intended.