How to clean the milk residue from the child's tongue?

You can take a small damp towel and wrap it around your finger and use it to wipe the tongue. The milk residue should come off. At least most of it. If find that it is difficult to come off, it could be milk thrush. The white coating on your child's tongue doesn't come off easily if it's thrush. You will need to consult a doctor in that case.