when to tell the kids

How far along did you wait to tell your kids you were pregnant? We are a blended family my boyfriend and I each have a 6 year old. We want to tell the kids but I don't think I am ready. I fear when I tell my son he will tell his dad which I know would create a little drama that I'm not ready for. I'm already 12 weeks and I'm thinking maybe 20 weeks would be a better time but at the same time my belly is getting very noticeable.