Missing s's

Hi there!

I've got a question: my big little one keeps missing the 's' of its the first letter and is followed by another consonant. E.g. She says "top" instead of "stop", "nake" instead of "snake" etc, with one exception: she says "sim" instead of "swim".

I don't have any idea of why, a couple of times we made it into a fun game of saying words slowly and with funny faces and she could say them perfectly.

(Our family is bilingual, but we all speak both languages.)

Do you have any idea how to get her to realize she needs to add all those s's?