Cartoon characters of the week: Daffy Duck and Sylvester the Cat.
These 2 have bad lisps! Did their mom's let them take their pacifier too long? Is it the anatomy/physiology of the oral cavity? I wasn't there so I can't say for sure. Well, don't panic if you 3, 4, or 5 year old has a lisp....it is most likely going to go away.
Do they always go away?
Can they always be corrected?
In my professional opinion.....it depends.
Anyway, if you want to seek private speech therapy, go for it. Don't expect that your school speech therapist will be able to provide therapy for this though. It is very, very, very hard to justify that there is an adverse impact on their ability to access the curriculum because of a lisp. That's not to say you shouldn't seek therapy from an outside agency.
In the meantime, teach them when they are older than 5 to keep their bird in the cage (tongue behind teeth). You can grab a straw and teach them that the air should come out of their mouth in the middle like they are blowing through a tiny straw. You might want to use a coffee straw that you stir with.
Relax and just keep an ear on things.