I hate the phrase, I know my children best.
I accept sometimes it is true, but lots of times it is not. Some parents say this phrase whenever anyone tries to say that their actions may not be the best for their child.
Examples can include pushing a new partner onto your children when the child obviously does not like them. Or refusing to allow a school recommended investigation for SEN. Or defending a child who has bullied or hit another child.
Most parents love their children deeply and it can lead easily to seeing children and situations involving them through rose tinted spectacles. Sometimes someone else can accurately see something about a child that their parent can not see.