It's funny - because Anna knows her daddy, when he dressed up as a very scary vampire for Halloween she didn't even bat an eyelid. But Anna is 10 months old. Léon, however, who is older and therefore has developed imagination, ran screaming in terror when he came across Thomas in the hall. In fact Léon's imagination is currently so vivid that if Thomas puts a towel over his head while getting dried after a bath he almost faints in fear because Thomas has become a ghost.
Personally, I found him much more terrifying in Amanda's wig after a beer or two!