Words repeated

It's always funny when the phrases we use at home are repackaged and repeated, with the added twist of a three year old's comprehension. Max has been known to ask his parents if they need a hand in the loo. "Do you need any help?" he'll ask us through the closed bathroom door.  I've also heard him tell Dom he's "had a long day" when asked how the last 12 hours of play went for him.
Over the past year when Max has been trying to communicate with Oli (2 years his junior and yet to get his first word on the vocab scoreboard), quite obviously he has not been given any intelligible responses. I've tried to tell Max that Oli doesn't really understand.  It seemed to help a little with the frustration Max felt when his train track was mauled or his tower destroyed.
"When we tell him not to do that, he doesn't understand." I say.
I try to demonstrate by asking Max to put his finger on his nose. He does it beautifully.
"But look", I say, "if I ask Oli to do the same thing, he won't, because he doesn't understand."
Well, now that Oli is a little older, it's quite possible he might understand. He just might touch his nose.  Or he might answer me correctly when I ask him what a dog says.
'What does a dog say Oli? " I ask.  I try to encourage him by pointing at one.
"What does a dog say?"
I promise you, Oli does have a noise for a dog's bark even though it's not really a 'woof' and I may be only one who can recognise it.  On this occasion, Oli gives a baby babble that, I can't deny, sounds nothing like a dog.
I see Max looking at me.
"Mummy," he says with a knowing pause and a sigh that hints of despair at having to spell it out yet again "he doesn't understand."