Corrected entry: At the start of the film Arthur Spiderwick is in the house. It then states "30 years later" when the kids and their mother arrives. Later on in the film they meet Aunt Lucy who Is 86, who tells of how her father was taken, etc. Further along the film they meet Arthur who said he just left the house, and Lucy is 6 and a half. The boy explains she's 86 and a half, and Arthur is 125. If Arthur left the house when Lucy was 6 and a half, and the mother and kids arrive 30 years later, that would make aunt Lucy 36, not 86.
Corrected entry: Near the end when the mother finds out the fantasy creatures are real, Mallory gives her the head band with the stone so that she can see the creatures. During the fight the headband disappears but the mother can still see the creatures.