That sounds a bit odd?? I think the most concerning parts are if they ever used a real knife and if the "play" truly did include any sexual components. I think it also matters how much older than you they were. A group of 3 five year olds might not know any better, but it gets a bit weirder if there's more of an age gap.
Since it isn't currently causing you any distress I wouldn't worry too much about it. But if you start to remember more or the memories start to concern you, then you can always try to talk to a therapist about it.