Well finally decided to take my kid into an orthodontist at 8 and a half years and we are putting in a lower sublingual arch. Do they limit or impact facial growth?

Is there anything to be concerned about here and how I can better get him to understand proper tongue posture and all that? He was breastfed and as far as I know he seems to have pretty good tongue posture. The diagnosis our doc gave -

DIAGNOSIS
· Profile: Convex with retrognathic mandible
· Dental malocclusion: The upper and lower back teeth fit together normally
· Overjet: 3mm
· Overbite: The front teeth overlap excessively and are causing swelling on the roof of the mouth
· Maxillary arch length: Moderate crowding of upper teeth
· Mandibular arch length: Moderate crowding of lower teeth
· Periodontal attachment: WNL
· Minimal attached gingiva: NSF
· Dental abnormalities: Premature loss of tooth 7.3