Your example is somewhat flawed. If Bob readies an action to attack Alice as soon as she's within reach, and then moves within reach, it's just a normal move/attack sequence. No Readying is necessary, in which case Alice resolves her readied attack as soon as Bob is within reach and then Bob acts.
Let's go with another example.
Alice has the best initiative. Alice readies an attack if a foe enters a certain area. Bob has the next best initiative. He readies a spell if a foe enters the same area as Alice is watching. Then a foe enters that area. Who goes first?
The description of the Ready action (p. 192, PHB) doesn't state this situation specifically but it does imply some things:
First, the Ready action is used to "get the jump" on an opponent that is acting after you in the initiative round.
When the trigger condition is met, you get to go before your opponent because you would have gotten to go before them in the initiative order anyway.
If we take that and expand it to others who might be reacting to the same trigger, then multiple readied actions should be resolved in the order in which their characters would have normally acted in the initiative round.
So, Alice would go first, then Bob, then the foe if it survived.
So, my interpretation is that multiple Readied actions are resolved in initiative order (whatever that would have been).