I think its perfectly fine, as long as its not to scary , that it may frieghten the child once the party is over, simple monsters, that do not look very bad are okay .
A frightened person should look to the Lamb of God, Jesus, who takes away the sins of the world (John 1:29). By praying to God and asking Him to forgive our sins because of the payment of our sins by Jesus on the cross. Once we are trusting in Jesus as our Saviour, the Bible says we grow until we get to the point that the perfect love of God casts out our fear (I John 4:13-18). Pray to Him and ask Him to come into your life and He will, and He will help you with your fear.