Learn Spanish (Spain): El niño camina al parque. - The boy is walking to the park.

In Spanish, when the preposition a (to) is followed immediately by the masculine word for "the", which is el, they always combine to form the contraction al. It is mandatory to blend them together, so saying a el parque would be incorrect.

The English translation says "is walking", but isn't camina just "walks"?

It can be both! In Spanish, the simple present tense (camina) is very flexible. It can mean "he walks" as a general habit, but it is also commonly used to mean "he is walking" right at this very moment.

Parque ends in an -e. How can I tell if it is masculine or feminine?

Words ending in -e don't have a strict rule for gender, so you usually just have to memorize them. However, in this sentence, the contraction al gives you a big clue: because al contains the masculine article el, we know that parque must be a masculine word.

Does niño only mean "boy", or can it mean "child"?

It can mean both. El niño specifically means "the boy" or "the male child". If you were talking about a girl, you would change both the article and the ending of the noun to say la niña.