Does having a cross bite affect a young child learning to speak?

indeed cross bite will affect speech  there ar eamny easy wasy to fix this including the use of soft rubber mouth gaurds worn at night  to help correct this .

Yes, from an orofacial myology point of view there could be a few things going on. Firstly, prescence of large tonsil and adenoid tissue precipitates the tongue into a ‘fronted’ tongue position and often open mouth posture. Mouth breathing and habitual open mouth posture are also common with enlarged tonsils and adenoids, along with audible breathing and sometimes snoring at night time. This can  also commonly leads to excess saliva. Functionally, ‘forward’ tongue posture is most often seen during speech swallowing and chewing with enlarged tonsils and adenoids. Fronted tongue posture and function are best corrected via a course of orofacial myology as without intervention the habits do not commonly resolve on their own and can become long term habits which ultimately impact on development of the facial and jaw bones. Consideration of the impact of the tonsils and adenoids is also important.
I agree with Dr Marget, that use of a soft rubber mouth guard to correct the crossbite can be a very useful, however they do not necessarily correct the ‘tongue fronting’ muscle patterns that have developed.
Ultimately, if speech development is being influenced  by tongue fronting habits, you would want to be sure that speech is corrected in conjunction with other oral functions of breathing swallowing and chewing. Tongue fronting can most certainly impact a child's intelligibility and can occur on a wide range of sounds.
A good starting point would be to seek assessment of the speech difficulty from a Speech Pathologist who specialises in orofacialmyology. The fact that your child has already been having speech therapy for about 18 months,  is suggestive of other factors influencing his speech and language development and any further assessment would need to take into account the factors influencing late speech and language development including hearing, and look carefully at his rate of progress with therapy to date.

Interesting comments Sharon, thank you.

Myofunctional therapy can be effective, but is inefficient and largely dependent on the proficiency of the practitioner (of which there are not many in Australia).

I assume by ‘soft rubber mouthguards’, both you and Harry mean the Myobrace. If a member of the public please visit Myobrace.com or if you want to read the research, please visit Myoresearch.com

This appliance encourages proper tongue posture and function, as well as eliminating mentalis and buccinator activity from swallowing. This is all syngergistic with speech pathology goals.

i would say you have answered this a narrow palate  no rooom for tongue + = speech problems

Chiming in again on this question. The most common speech difficulties associated with cross bite are tongue fronting and/or tongue lateralisation. The first creates ‘lispy’ speech and the second causes air air escape from the sides of the tongue, distorting the sound/s and giving a ‘slushy’ effect. A 4 year old's speech should be largely understood by everyone that hears them, including non familiar people, even though they may still have a few developmental speech errors that can resolve in their own in time.  I hope this clarifies further on the ‘speech’ and cross bite issue. Speech intelligibilty is effected by other factors apart from cross bite but your Speech Pathologist will be able to help you sort these things out.

From an ENT point of view, having spent a lot of time learning from dentists and orthodontists, there is a lot that an ENT that specialises in this area can pick up on and as part of a team approach make things progress favourably. 

If the cross bite of the front teeth then yes as this affacts position of the tongue against the teeth during some sounds.