Both my kids have a very slight lisp, DS moreso than DD. I think it's beautiful. DD is top of her class has a tonne of friends, although you can hardly make hers out now. DH has a slight one until the end of primary. I tell you this not to minimise your concerns but that kids can thrive and grow with a minor lisp.
Having said that, I understand you have one and had issues growing up and don't want the same for your child. So I say based on that see a speechie.
How severe is your child's lisp?