Does anybody have any experience telling their step-kids (or children who don't live with you full time) about wanting to try for another little one?

Some background:
My husband has two children from a prior marriage, 13 (girl) and 10 (boy - almost 11). We have them every other weekend and they're truly great kids; the 13 year old is hitting those crazy teenage years with a vengeance, but I can still see the sweet girl underneath it all :). I've never been pregnant and this is my first marriage.

My step daughter used to ask me if Daddy and I were going to have a baby (this is a few years ago now, before we were engaged I think) and I would smile and ask her if she wanted us to, and she would respond with "yes, right now!" lol. She didn't understand the dynamics behind it then, though she will now. I hope she still feels the same way, though I understand that there's other forces at play and she or her brother may feel replaced if we're lucky enough to conceive -- I'll try my very best to make sure that they don't ever feel that way. It's going to be a balancing act for sure.

They each have a bedroom at our house (I purchased it prior to our relationship), and I've worked hard to make sure that neither will be displaced -- we have a room near the master that's functioning as a guest room right now, but can be changed into a nursery. My husband and I hope to start TTC in late summer/early fall; I'll be turning 33 then.

So, as August approaches do we start thinking about sitting them down and letting them know what we plan? Or is that too early, and should we wait until we're (hopefully!) actually pregnant and far enough along that it's safe to start spilling the beans? Am I thinking about this too much? lol Beyond saying "we'll be lucky if the baby is as awesome as you two are" I don't have too much in the way of ideas. I know that I'll be sweating bullets through that whole conversation.

Thanks!