Learn Italian: Il bambino ha la bocca piena di pane. - The child has his mouth full of bread.

In Italian, when talking about body parts, you usually use the definite article (il, la) instead of a possessive adjective (suo, sua) if it is obvious whose body part it is. Since the child is the subject, it is understood that it is his own mouth.

Why is the word for "full" spelled piena with an "a" at the end?

Piena is an adjective describing the mouth (la bocca), which is a feminine singular noun. The base adjective pieno changes its ending to match the noun it describes. It does not matter that the child (il bambino) is masculine.

Why is it just di pane (of bread) instead of del pane (of the bread)?

The expression pieno di means "full of". When you follow di with a general, uncountable substance like bread (pane), you usually drop the article. You only use del (which is di + il) if you are talking about a specific loaf or batch of bread.