This is symptomatic of a larger problem; that's because history teaches us that Mike and Lizzie are suckers for dares; their need to fit in and be liked is a means by which people who never cared for them and never will can use them as a cheap sort of entertainment. Sadly enough, this love of cheap laughter is accompanied by a love of not admitting that if they'd remained silent, their chump would have avoided getting into easily avoidable trouble. According to the army of Pontii Pilati in short pants, since it was a Patterson's hand that did something stupid, it was all on them for listening to the advice of a buck-passing arsebucket.
The reason that they get away with this sort of shell game is because Mike and Lizzie are suckers in every other part of their lives. We know that the Patterson children are willing to believe in any God-damned thing except the truth; that's because they were raised wrong by imbeciles. Most of the time, Elly is less concerned with how the child feels (usually like crap) and more with how big an inconvenience the misbehavior causes her.