Sleeping beside someone; to lie down together next to another person, often used for comforting a child or describing partners sleeping together.
子どもが怖がっていたので、母親が添い寝してあげた。
The child was scared, so the mother slept next to them.
友人と旅行したとき、宿で布団が足りず添い寝することになった。
When traveling with a friend, there weren't enough futons at the inn, so we ended up sharing a bed.
ホテルの家族向けプランには子どもの添い寝無料と書いてあることが多い。
Family plans at hotels often state that children can sleep for free when sharing a bed with parents.