Head Canon 1#: Eugene is narrating Tangled because he's telling the story of how Rapunzel and him got together to their kids.

"There's the story of how I met your mother," Eugene told Rupunzel's and his kids. "Now it's time for bed."

There were cries of "Tell it again!" and "I don't want to go to bed!" from their kids.

"Now all of you promised to go to bed after the story and we always keep our promises," Rapunzel told them.

"Yes, mom," the kids said in unison and they walked grumpily.

"Be good and you can story time again some other time," Eugene said.

"Do you promise daddy?" their daughter asked.

"I promise," Eugene agreed. The kids cheered and then they continued off to bed. Eugene and Rapunzel followed after them to tuck them into bed.

I figure they have at least two kids.