I get you. But I honestly still think it will be quicker to stay on your current server and unlock them as they are realeased. That way they tend to get moved around the various shops pretty frequently so you maximise your opportunity to unlock them. Rather than joining an existing server and being forced to do pretty much all of the elite campaign (which you will have to unlock, and that takes time) in order to unlock pretty much all of the older characters.
To show you what I mean, on my server (which has all of the characters) in order to just unlock Buzz (who your son will presumably desperately want with Toy Story 4 about to be released) you need to collect 30 shards which are only available from chapter 13 of the elite campaign. In order to unlock this you first need to complete chapters 1-13 of the normal campaign and then each fight costs 24 stamina, with something like a 1/3 drop rate. In other words 2,160 stamina (ish) once you have actually gotten there.
