Dad said that he's having dinner with some work friends tonight.
Dad said that he's going to have dinner with some friends from work tonight.
Dad said that he's going out for dinner with some work mates tonight.
「パパ」には、dadの他に、daddy や papa などの呼び方があり、daddyは小さい子どもがよく言う言い方です。
「同僚」は、colleagues や co-workers などと言い換えが可能です。
もし、さらにわかりやすく伝えたいなら、so he's going to be home late(だから、パパは帰りが遅くなるよ)などと付け加えましょう。