Hi!
I've travelled with a 2 month old baby in C class. Luckily he slept pretty much all the way. Wasn't an issue on boarding or not.
Obviously the crew will appreciate it if non-rev kids are not running around up and down the aisles or keeping anyone from sleeping. So just keep an eye on that.
No standby allowed in F class for people of any age anymore at LH, even our own staff.