we already have a 4 month old kitten and we got another one a couple of days ago (shes 10 weeks, the new one). i thought it would be nice for poppy (the older one) to have someone to play with. the new kitten is not really bothered by poppy and will play happily with her, but poppy charges at her and sinks her teeth into her neck and holds her down, this obviously makes the new kitten squeal. poppy doesnt arch her back or hiss or anything else, shes just gets her in the hold like she would if she was killing her prey.
i have had to separate them as the new kitten is still settling in and finding her way around. i would be really grateful if anyone could give me any advice on how to handle this. i really want this to work as i adore both of them.
thanks in advance.

If there's no fluffing up and nasty language between the two of them then it's very likely that this is actually a game (which may or may not be going too far and risking hurting your little one).

If left to do it what does Poppy do? What happens after she's jumped on the little one and chewed her for a bit? If it's a game it's more than likely that the next step would be some fake disembowelling with back claws and a bit of mutual ear washing. Have you left her long enough to see whether the little one actually gets hurt?

When my Merlin was a tiny baby I used to get very concerned when my grown up cat Tizzy jumped on her but when I just stood and watched one day I saw that Merlin's squealing was due to being held down and having her tummy washed. She was totally unharmed but a bit damp afterwards and soon learned to play along and enjoy the game.

i have left them for about 10 mins and poppy is still holding her down and shes still squealing. i havent seen poppy washing her yet. they have eaten stood next to each other, but when the new kitten was eating earlier, poppy still charged at her and pinned her down. as soon as i let poppy out when i have separated them, shes looking round for her to jump on. i dont believe poppy is being nasty as she hasnt been nasty at all since we got her, i have 2small children and poppy is great with them and loves playing with them. i think she is playing but is a bit rough.

our kitten tries to do this to our older cat, he is constantly charging and tries to get his arm around the other cats neck. There is lots of leg kicking (we wondered what that was for, it looks a little energetic at times). Surprisingly its the older cat who is usually squealing. I think your smaller kitten will soon be getting its own back on the older one as it gets bigger.