Let me preface this by saying that I am a big ERF supporter. My kidlets RF to at least three, four if at all possible, and longer if it's feasable.
First of all: good job keeping your daughter RF. She's much safer that way.
What seat does she ride in?
Is the not-yet-4yo still in a harness?
As far as the not-yet-2yo goes, unfortunately, they are his parents and it's up to them to make decisions regarding his safety. Although he would of course be safer -- 5x, although as Darren points out we don't know what that's relative to -- he is reasonably safe in a FF seat that is properly installed, properly used, and tethered. If talking to them about ERF is not getting you anywhere, it may be best for you to concentrate on proper usage of the seats they have. If you see any obvious misuses, get them to correct them, or better yet, have them take their kids to a tech to get the seats checked.
If you feel like you need to have another word on the subject, you could point them towards Joel's Journey. I believe the website is joelsjourney.org . I cannot get youtube to work, but Joel's video on there is "rear facing or forward facing?" and it tends to be very effective. There should be other RF vs FF tests on there, the overhead ones tend to illustrate the point the best.
My suggestion would be to compose a reply, say what you need to say, and then let them know that it will be your last word on the subject. If you continue to needle and pick about it, you risk losing any credibility you have, and they won't listen to you about CPS at all. I had this situation with a child in my care -- his parents won't use a booster. (He's 6.) I finally had to let them know my thoughts on it, explain to them why, and let them know that it was my last word on the subject. It didn't change anything, but at least I feel like I tried, and they are still comfortable coming to me about CPS-related things.
It's unfortunate that parents often believe outdated information or untruths, but it is what it is. If you're not going to change their minds, it's not worth ruining the relationship over. If they ask about your daughter, state what you know, but just as you won't budge on what they want you to do (FF,) they likely will not budge on what you want them to do. It's a hard thing to come to peace with. Good luck.