Picky eating

My children have started telling me "I don't like to eat that" when I serve them foods that they've always eaten in the past.  I'm hoping this is just a phase.  I've always heard it takes 10-15 times or so of being served a new food before a child will sometimes try it.  I'm not sure how this applies to food they have eaten before.  I try to just ignore them when they say it, and serve it to them anyway.  Today my 4 year old told me "I don't like grilled cheese.  I won't eat it." I ignored her and made it for everyone anyway.  She ate half of it, which I considered a victory.  I'm hoping they don't get in the habit of saying they don't like something and never eat it, only to find out as an adult that, what do you know, they really do like it.  I'm pretty sure that's what happened to me and tomatoes.  Any suggestions?