T-D vs. K-G sound discrimination
The t-d sounds are made in the front of the mouth, with the tongue tip touching the alveolar (gum) ridge behind the upper front teeth. The k-g sounds are made with the BACK of the tongue lifting to touching the roof of the mouth. Children often substitute the t-d sounds in place of the k-g's. Try using this to help them hear and see the difference on words with only that sound difference...