Non Arabic speaker here: نعلمهم What's wrong with this answer?? Thanks. سلام!!
Your answer is more like "we have knowledge about them" and is not usually used when you want to say you "know" somebody. However, it can be used when you want to say "I/We know about somebody or something". For instance, "I know about her" would be "أنا أعلم بشأنها". Please feel free to ask me if you have any more questions :)
ما اعرف ليش بعض شباب تتفلسف اكثر من لازم الكلمة صحيحه رجاءا لا تشوشون على مبتدئين