When the baby sees adults eating, his/her mouth starts chomping away. This is a sign he/she is ready for solid food.
When the baby watches adults eating, he/she copies them and starts moving his/her mouth. This is a sign he/she is ready for solid food.
「赤ちゃんが、大人が食べているのをみて、口をもぐもぐし始めたので、離乳食の合図です」= When the baby sees adults eating, his/her mouth starts chomping away. This is a sign he/she is ready for solid food. / When the baby watches adults eating, he/she copies them and starts moving his/her mouth. This is a sign he/she is ready for solid food.
その「もぐもぐする」が英語で「to munch」か「chew」と言います。
以下は例文です。
赤ちゃんが、大人が食べているのをみて、口をもぐもぐし始めたので、離乳食の合図です ー The baby started to muzzle when he saw an adult eating, it’s a sign we should start feeding it baby food.
うさぎがもぐもぐ草を食べているのを見た ー I saw a rabbit munching grass.
参考になれば嬉しいです。