You know that you don't have to say anything tonight?
If you can't be around him, and you want thinking time, you can tell him that your mum is unwell and has asked you to stay through the weekend.
It's OK to buy yourself time, if you need it.
Getting financial info as a priority does depend a bit on your situation. My XH also used prostitutes - I knew all about both of our finances and knew that he couldn't hide things. For example, he was salaried so there were no self employed earnings to doctor! So yes, take copies - but don't panic!
It's fine to text him a copy of your evidence and say "do not fucking dare come home until I decide what to do" then ignore him.
It's also fine to keep your powder dry and talk to a solicitor.
My situation was "easier" because I'd had suspicions so the final proof wasn't a shock.
I don't know if this will help, but my daughter was also 4, 4.5 when I told her we were divorcing. I actually think it's a good age if you have to split. Old enough to have secured a relationship with both parents. But young enough that they're generally OK with change, and don't ask questions! I'd have hated to split with a young teen saying "so did one of you cheat?". At 4, separated life is quite quickly 'normal', in my experience.
With regards to what you say to him... what do you actually want to know? Bearing in mind that he'll lie, because that's what he is. Expect the "I cancelled it / didn't show up" bollocks 
You don't HAVE to listen to his shit.
Btw - my solicitor said that there is often a window of opportunity whilst they're still feeling their own pathetic version of guilt and woe is me I'll change crap, when you can have more productive discussions about the split.
I found my XH was a bunny in the headlights when I was saying "you will have this access, I will move here, you will pay a share of...".
Good luck. I'm so sorry 