Young children and a 4 month old kitten

Please don't take this the wrong way, but your post is very difficult for me to read. I can't tell if you're abbreviating some words or typoing.
I think I got most of the gist of what your post said, sorry if I miss some details.

First: Make sure to teach the kids to keep their faces back from Olly. Kittens will even play by swatting at each others faces, so even if he was only playing they'd be at risk of getting scratched.

Second: Is he neutered yet? If not, you need to get this done. Yes, I know it's around the holidays and funds are usually a little tighter.
Third: Do you keep his claws trimmed as short as you can? Or if his claws are large enough, yet, have you looked at the smallest size of Soft Paws? I really suggest you try using them since you have such young children and such a young kitten in the house.
While some kids can shrug off getting scratched (I grew up with cats and played outside, when I was 3-5 scratches were nothing to me. I fell face first into a huge rose bush when I was 4 ).
Do make sure to keep an eye on the scratches and if the kids get sick in the next few weeks get them to the doctor just in case.

Olly's obviously one of those rough and tough type male kittens. They play hard and can be a real pain be cause they don't know their own strength sometimes. Really the best thing for them in a slightly older cat companion that can take that rough play and give it right back to them. It's through receiving that sort of play that a kitten learns that scratches and bites hurt.

I suspect the kids in general, being kids and not able to help it, are still sending some mixed signals, too. Moving a bit too fast, getting their faces too close, staring Olly in the eyes, and just being loud. All of these things can trigger some cats and kittens. You're just going to have to monitor and train everyone (kids and cats, and learn some new things yourself).

For the 5 year old: Young kids learn by example. If you make a big deal out of something then they'll make a big deal of out it. Have you ever seen some little ones that fall down and bump their heads and just get right up and go back to playing? And then the others that will fall down and start crying - with a parent that quickly rushes in and makes a big deal over the little bump? It's like that.
You're going to have to be calm and sure about this so that she'll calm down. If you're careful, but not paranoid and jumpy, the kids (and kitten) should pick up on your mood and actions.

Sorry to hear you are having this problem -kids and cats can be a bit volatile with each other!

I would talk to your kids and tell them that Ollie doesnt understand what they are saying to him. You can go on and offer to teach them some 'cat language' so he will know they are only trying to be friends. Then show them how to sit and wait and let him come to them, not approach him, because that means 'get out of here' in cat language, whereas if they sit still it means 'I want to get to know you' and he is more likely to come towards them. Then make sure they do not look him in the eye because that means 'I don't like you - go away' while slow blinking at him means' I like you and I won't hurt you'. This may sound complicated but do it slowly and it is not really, I did it with my daughter when she was a toddler and she has grown up to be the most empathetic cat person I know.