All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.
If something happens to a person during prayer and he wants to attract the attention of those around him, be they young or old, he should say Subhan Allah. For a woman, she should clap her hands following the Sunnah of the Prophet (PBUH) who said: " (Behold) when anything happens in prayer, say: Subhan Allah, for when you would utter it, it would attract the attention, while clapping of hands is meant for women." {Related by Bukhari & Muslim}.
If the praying person wished to attract the attention through saying Allahu Akbar or Sami' Allâhu Liman Hamidah Loudly, then it is permissible. When saying them, he should solely intend remembering Allah or making remembrance, in addition to attracting the attention of a certain person. This way, his prayer remains valid. However, saying the aforementioned phrases for the sole purpose of attracting the attention renders his prayer invalid according to the position of many jurists, and negates the concept of "Qunut" which Allah, may He be Exalted, Has Commanded in prayer where He Said (What means): "and stand before God in a devout (frame of mind)." {Al-Baqarah, 238}.
Al-Khateeb Al-Shirbini {May Allah have mercy on him} said: "The aforementioned clarification applies to Saying Allahu Akbar or Sami' Allâhu Liman Hamidah Loudly. If they were mentioned to remember Allah and attract the attention, then the prayer is valid. But, if they were solely mentioned to attract the attention, then the prayer is invalid. And Allah Knows Best.