Eileen... well, she'd freak at first but since she isn't having the "trapped somewhere" and "Walter in my brain" stuff going on (presumably) I think she'd be in a better place to deal with it and not freaking out as much eventually.
Well, Frank talked to Alessa some and in my fanon (and canon to a small degree) he already realized the baby and the umbilical cord were connected with what happened to Henry. Plus Alessa told him the baby (well, who he figured out was the baby) ended up being named Walter. And since the name Walter Sullivan was in the news way before, and in my fanon Frank reserached Silent Hill... I'm thinking when he sees Walter and knows his real name he'll know enough to freak out, even in Walter doesn't reveal the obvious yet. If that makes sense, at least? But Walter may say enough when/if he shows anyway.)
(Hey, that icon is... cool and interesting! XD Do you have the full size of it? I'm curious.)
That reminds me, I need to upload Eileen icons... I hope you don't mind if I use some of the ones you gave me for JF.