Thumbs and sucking fingers can have an impact on the mouth and jaw of children at the age of 2 years. Sucking puts pressure on the soft tissue of the roof of the mouth, and on the upper jaw side. The pressure can cause the maxillary to be able to narrow, which prevents the teeth from tightly tightly when the jaw is closed. When a child sucks their thumb until after they lose their baby teeth and permanent teeth they enter, the appearance of "buck teeth" can develop. You can find thumb sucking guard via https://www.amazon.com.au/Age-2-7-STOP-Thumb-Sucking/dp/B00VQU2DZS.
Braces are expensive improvements for this problem. However, if a child stops the digit sucking before their permanent teeth eruption (usually around the age of 6) changes in the mouth and teeth can complete themselves and do not require braces to improve.
The severity of physical problems originating from the habit depends on how hard a child sucks their thumb. If they only rest their thumbs in their mouths without too much sucking too much, it will likely there will be fewer problems than if it is an active movement.
Children who make a lot of suction when sucking digits (you might hear "pop" when digits out of the mouth) are more likely to influence the pattern of oral growth and teeth compared to children who just put their mouths or suck gently.