- I love watching them play in the park.
- I love watching them playing in the park.
Which one is correct or natural if both are correct? Why would you use the bare verb over the gerund in sentences like this?
The first option is correct.
The reason for that is the second sentence is actually two gerunds nested inside of each other. Let me show you what I mean.
I love watching them play in the park.
The bolded text is the direct object of the sentence. "Watching them play" is a gerund phrase. "In the park" is just a prepositional phrase, and like most, it can be removed from the sentence without taking away anything from the grammatical structure or correctness. "I love watching them play." is also a valid sentence and a complete thought.
I love watching them playing in the park.
"Playing in the park" (or I suppose more technically just "playing") is a gerund phrase on its own, apart from "watching them". So what you are effectively saying is "I love watching them [noun]." As you probably know, nouns cannot be used as verbs.
I love watching playing children.
This is a correct sentence because "playing" is a participle used to describe "children," but it is a little awkward to say so you will not likely hear or see it in normal conversation. A more common version would be "I love watching children at play." I just wanted to play devil's advocate and show you what it would take to make a valid sentence with both "watching" and "playing" in the sentence, while preserving the meaning.