My dad recalls an episode of a sci-fi show that either takes place on Earth or involves humans going to a different planet. Anyways, in the episode, humans encounter giants and engage in battle. The humans are finally victorious only to realize that those giants were just the children and now the adults are coming. My dad doesn't recall which show this was. I've looked at The Twilight Zone and The Outer Limits. They don't have an episode like this. Anyone know what it is?
While you say it isn't The Outer Limits, it certainly sounds like an episode I remember from the 90s version of the show. The episode was called "Relativity Theory"(Season 4, episode 6).
The aliens the humans kill turn out to be the equivalent of alien boy scouts on a camping trip, which is why they only have primitive technology. The world they are on is alien to both species. At the end, the parents turn up in a much more advanced space ship with an intent to punish the human race for killing their children. They aren't giants, but the gist of the story seems to be the same.
A clip from the episode.