The correct option is
C Besides
Option A: The word, 'beside' means 'at the side of' or 'next to'. Given this meaning, this preposition is not appropriate to be used in the given blank.
Option B: The word, 'besides' means 'in addition to' or 'apart from'. When used in the sentence, it means: 'In addition to children, parents also attended the party'. As it completes the sentence meaningfully, option B is correct.
Option C: The word, 'alongwith' does not exist in the English language. 'Along' and 'with' are two separate words. 'Along with' is a phrase which means 'in addition to someone or something else'. Hence, option C is incorrect.
Option D: The word, 'with' means 'accompanied by'. However, the sentence, 'With children, parents also attend the fun party' is not the correct usage. The correct usage is: The parents also attended the fun party with the children. Hence, option D is incorrect.