This is the 5818th most frequent Arabic word.
مصابا
Afflicted, injured, or stricken.
Here, 'مصابًا' is used as an attributive complement describing the state of the man, indicating that he was affected by a condition (pain).
كان الرجل مصابًا بالألم الشديد بعد الحادث.
The man was severely affected by pain after the accident.
In this sentence, 'مصابين' (plural form) describes the children, signifying they were in a state of joy, positively 'affected' by it.
شاهدنا الأطفال مصابين بالفرح أثناء الاحتفال.
We saw the children affected with joy during the celebration.
'مصابين' is used as a noun participle referring to people who are injured, derived from the contextual meaning of the verb 'أصاب' (to afflict).