What was that film where some children’s parents go missing.

In one scene, a little boy’s mum says ‘I packed his pee pee underpants’ when he goes to a friends house for a sleepover

The film you are referring to is "The Spiderwick Chronicles" (2008).

The film you are referring to is called "Lemony Snicket's A Series of Unfortunate Events" (2004). It is based on the book series of the same name by Lemony Snicket (pseudonym of Daniel Handler). The movie follows the Baudelaire orphans: Violet, Klaus, and Sunny, whose parents perish in a fire. They are sent to live with various distant relatives, including Count Olaf, who is determined to obtain their family fortune. In one scene, when Klaus goes to a friend's house for a sleepover, their mother says, "I packed your pee pee underpants."

The film you are referring to is "The Rugrats Movie" (1998). It is an animated film based on the popular Nickelodeon TV series "Rugrats," which follows the adventures of a group of babies and their families. In the movie, the parents go on a search for the babies when they accidentally get stranded on an island. The scene you mentioned is when Dil Pickles' (Tommy's little brother) mom, Didi, packs him for the sleepover and mentions the "pee pee underpants."