Richard + Lilli smarter?

You're entitled to your opinion of course.

But when you say, "Emmeline was pretty silly not to suspect something about her growing size especially since she had to have noticed that something was moving in there," I think you either haven't seen it in a very long time or weren't paying very much attention. Of course she noticed something moving in there -- and definitely doesn't think everything's just fine -- there's a whole scene about it.

Right after a nighttime scene where Richard takes away a coconut and says "Will you stop eating, you're getting fat," and an early morning scene where Richard sees her throwing up and looks (in my opinion) worried, there's a nighttime scene where she's sitting upstairs in their hut finger-combing her hair. Richard comes up the ladder and approaches her, she shrugs him away, he makes a (in my opinion) frustrated-sounding sigh, and to quote word-for-word:

Em: I'm sorry Richard.
Richard: You didn't wanna all day yesterday either. Whatsa matter Em, don't you love me anymore?
Em: Yes, I love you more than ever, Richard.
Richard: Then why don't you wanna do it?
Em: It just hurts right now, that's all. When it stops hurting we'll do it.
Richard: When is that gonna be? [Em doesn't answer.] I don't understand, Em. Why does it hurt?
Em: I don't know. I don't know anything. But if you touch my tummy right now you can feel it.
Richard: Feel what? [Em positions his hand.] How'd you make your tummy move like that?
Em: I'm not doing it.
Richard: It's not doing it by itself.
Em: Yes it is.
Richard: There! I felt it again! What's making it do that, Em?
Em: I don't know.

Leaving out my opinions about her looking scared, sounding scared, and sounding almost scared enough to cry, it's still obvious they know something's very not right.

Personally I think she's smart and intuitive to associate something moving in her tummy with what hurts where, instead of associating it with the other weird things that also involve her tummy, the constant hunger and the frequent throwing up (although those do, less directly, have to do with the same thing).

The very next shot is her looking at her reflection in a pool, feeling her now-huge belly with her hands and inspecting its size. True, she doesn't suspect there's a baby inside her -- last she was told they get dug out of a cabbage patch, and in some sad cases the doctor has to plant them again in the cabbage patch so they can grow into an angel. But it seems clear to me she's very aware that there's something drastic happening to her body.

"Just my opinion but they should've been able to grasp some of what was happening to them."

My opinion is that staging themselves a wedding ceremony (looking at the wedding slide and then dancing together in more clothes than they ever wear) soon after they discover sex shows a remarkable intuitive grasp of what was happening to them.

If you read the books some things about the story will make more sense. The book sequels have little in common with this movie other than the fact even though the original end with Arthur Lestrange asking if the original main characters and the baby is dead or alive despite being told they are asleep in both the second book and the movie sequel the lovers from the first movie and book are newly dead in the second book the first thing said in the book is the young lovers has just stopped breathing while the child is still relatively healthy and he is the main character of both the sequel to the movie and the other two books in the series. Even though in the first movie it does not say they know what makes a baby in the book it implies they have some idea after the child is born one passage from the book about the baby says the young lovers would talk about his creation without a blush.