How many children do you have? It would be traumatic enough losing you, and they'd probably need/cling to each other if you died, so if at all possible, I wouldn't split them up.
I chose a cousin who I'm close to, and my oldest/dearest friend. Both are mums, so they know what is needed in being a parent/surrogate. Both also have values and beliefs similar to mine, so I have some idea of how dd would be brought up. I think my sisters (childless) and oldest ( also childless) friend were initially unhappy with my decision (as both kindly volunteered), but I didn't think they could be thrown in at the deep end...so to speak.
I was asked to be a designated custodian by close friends. After much thought, I told them they should probably go with someone else as their son probably would suffer having to move hundreds of miles away from home/friends. It was a tough thing to verbalise, because I would do almost anything for those friends, and of course was afraid of offending them. But, they conceded that I was probably right when they'd given it more thought.
It's a very hard decision to make, so good luck.