The prayer is valid unless you do something that nullifies it such as laughing out loud, talking, eating, etc. However, the lesser the Khushu the lesser the reward.
https://www.youtube.com/watch?v=RtR_XuFz6WY

My question is on one’s concentration in salah. If one has bad khushua and while praying his mind is somewhere else, is his salah still valid?

Prayer | November 17, 2015 3:41 am
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I would like to ask, if someone prays Maghrib or any fard salah then realises later that they prayed one rakah less. Do they have to pray again?
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My daughter is 6 yrs old and have learnt to pray and can pray on her own. She has memorized half of the juz and I have memorized 2 juz. We pray at home in jamaa but she sometimes insists on leading the prayer. Can I let her lead me in prayer so that she feel happy and get confidence?