We told our 8 yr old the other day. He grinned like a nut and has been making sure that I am eating all my meals and pressuring me to exercise hehe.

He's so excited. We looked at pictures of what the baby is like at this point, etc.
I told my 6 yr old yesterday. He kinda gets it, but not really, I think.
The 4 yr old has heard about it, but doesn't really seem to care. lol.