Halloween has a theme around death (e.g. ghosts, skeletons, graves, zombies) and evil (e.g. devils, witches).
A baby in a pumpkin outfit is cute. Kids playing games and having fun is good. Those are examples of us (e.g. everyone, faith or none)celebrating good things.
Real death and evil are repulsive to us (e.g. again, everyone). Death feels wrong and soul wrenching. Evil is seen in pointless acts of violence and subtle exploitation etc.
The Christian faith is all about Jesus becoming a sacrifice for evil because if God was going to be fair, he couldn't just let people off - there had to be a death - either the person responsible or God himself in human form (a.k.a Jesus).
So, as people who believe someone has died for them to able to be forgiven, Christians might see the themes of death, evil, good and life from this living spiritual perspective.
That means some Christians will opt out of Halloween, but some may choose to be involved - safe in the knowledge that colouring in a ghost isn't going to undo Jesus' love or power.