Nope, I NEVER underestimate my young, language-delayed children's ability to understand. Language development can be a tough thing to test with all the counterparts and complications including receptive and expressive language, oral motor functioning, muscle tone, articulation and phonological components and more. Different levels of ability and/or issues in any one or more of these areas as well as the tester/testee relationship significantly impacts the ability to accurately measure language development. My children are expressive language delayed. That is, they don't express themselves -- specifically via speech -- very well for children their age. Actually, they express themselves VERY well. They are full-body communicators -- using speech, sign language, pointing, props, acting out like charades and dragging me to whatever it is they want me to see or understand. Whatever it takes. But, given some of their issues include oral-motor planning (getting their mouths to do what their brain is thinking/wanting to say in a way that is recognizeable by the listener), low facial muscle tone (an "o" is harder to produce so they don't do it so often), and articulation (making it sound the way everybody else says it... understandable to the general public not just Mom). With all of these expressive language issues, receptive language becomes a bit tougher to test too. Often because the tester doesn't understand their response. But, let me tell you, as I said before, I have no doubt that my children with Down syndrome are absolutely able to comprehend our language... They even understand the subtle nuances of our language...
For instance, they know and understand way more than just the dictionary definition of a noun like "home".
We went camping last weekend. On day two of our trip and at a particularly fatiguing point in his day, Michael asked me longingly, "Home?" Said with just the right inflection to let me know that he was asking whether we were going home now so he could finally rest his weary camping bones. With a kiss and a hug, I responded, "Yes, we'll go back to the camper and rest". He leaned back down in the stroller for the walk and meekly mumbled, "nooo". I thought then, incorrectly, that he was fighting sleep. I meandered back to our campsite hoping he'd fall asleep en route... to no avail. Still wide awake, I took him inside the camper and plopped him down at the table offering him his favorite comfort-foods, apple juice and [Tostito] chips. Even with his beloved chips in hand, he more emphatically pleaded, "HOOOME?" Again, I acknowledged with a hug and explained that the camper was our home for the next few days... He hesitated -- briefly considering my answer and studying my face for better understanding (me of him, not vice verse). Then, with a determined squint of his eyes he said, "No.... HOUSE!"
He wanted to go home to his house not home to his camper. He understood the whole time we were there that the camper was our [temporary] home. I know because he used it like a home-base. Always wandering about (loosely supervised with a "Let's see what Squirt does flying solo" approach) and always coming back to OUR camper.... Home! But, when he said "home" in the pleading and different way that he said it, the camper is NOT what HE was trying to express! He knew what I meant -- we were headed for the camper -- and he knew I wasn't getting HIS meaning. So, he thought about it and finally expressed himself accordingly. In a way I could not possibly misunderstand!
They "so totally rock, dude!"