Gary and Katie are both 18. They are on their first date, having a picnic in the park. Katie looks over at a family nearby. A boy who is about 2 is eating an ice cream.

Katie: ahh, look at that baby, he's so cute.

Gary: yeah.

Katie: they're so lovely at that age aren't they.

Gary: I suppose.

Katie: oh I want one.

Gary: what?

Katie: I can't help it, I want one. Do you?

Gary: maybe one day.

Katie: well I want one now.

Gary: are you sure? I just don't think I'm ready.

Katie: well I am, and I'm having one with or without you.

Gary: look, can we at least talk about this.

Katie: I don't see what the big deal is.

Gary: It's a big decision. I just want a bit of time to think about it. It's a big commitment having a baby.

Katie: a baby? I meant I wanted an ice cream!!

Very good, but first 3 lines need to mention the ICE Cream and not a specific reference to the baby, otherwise it doesn't work, a she was clearly talking about the baby (you can't say ice cream is cute at that age! ) no massive rewrite just first 3 lines something like:

Katie: ah, look at that Cute baby over there with the ice cream!

Gary: yeah, Cute.

Katie: Makes me want one!

Gary: Yer I suppose.

Katie: oh I want one.

Gary: what?

Katie: I can't help it, I want one. Do you?

Gary: maybe one day.

Katie: well I want one now.

Gary: are you sure? I just don't think I'm ready.

Katie: well I am, and I'm having one with or without you.

Gary: look, can we at least talk about this.

Katie: I don't see what the big deal is.

Gary: It's a big decision. I just want a bit of time to think about it. It's a big commitment having a baby.

Katie: a baby? I meant I wanted an ice cream!!

I think the first start was OK but you I think you need to be more obvious to the viewers that she wants an Ice cream. Maybe she could say she wants an ice cream but he doesn't hear her.
I think you could go much further too, but I like over the top stuff.

KATIE: There's a van that comes round near my house that sells them.

GARY: What?!

KATIE: The man will give you a double for two quid.

GARY: Twins?!?!

KATIE: Or the supermarket does them too. In the freezers in the back.

ETC

I think it would also be funnier to leave them still arguing rather than have the reveal.

I think it's too much dialogue for such a simple punch.

Also the dialogue isn't confusing enough.

Ice creams and 2 year olds both; make a mess, give you a headache, are very sweet etc.

I am afraid I rather saw the reveal coming, adn I am asuming I wasn;t supposed to. It is an exchange that might work with established characters in a sitcom but there is not really enough to it to make a stand alone sketch.

The suggestions made take it rather a long way from your rather sweet initial exchange, but they are along the right lines that in a sketch you need think beyond the initial inspiration and play with the idea a bit more.