Well, this thread has sparked a lot of replies in a short amount of time!

I think the phonology of the language should avoid consonant clusters
entirely (credit due to yuri).  So we would have a language with a very
simple CV[CV[CV]] word structure.  Furthermore, it might make sense to have
a very limited consonant inventory with only one or two consonants per point
of articulation (one bilabial, one glide, one alveolar, etc) possibly
supplemented with clicks, as those are rather distinct.  Also I envision
having three vowels: one close, one mid, and one back.

It sounds like there is some evidence that invariant word order is easier to
parse, so I'd expect to keep that too.  I also think that tenses in the
strict sense should be avoided, using compound tenses rather than
inflectional tenses.

I'm not out to create an auxlang, I'm just thinking how best to stack the

As for why one would want to teach a small child a conlang, I'm not sure if
there's any specific reason behind it.  My understanding is that it wouldn't
affect their natlang acquisition because all but one person the child
interacts with would use the common language of the society.  I expect, as
some have pointed out, that either the project would only go so far as the
child would choose to stop participating for pragmatic interests. It might
be a case like some seen in bilingual families where the parent says
something in one language and the child replies in the other (in America,
for example, the parent says something in Korean and the child responds in
English, I know at least one family that does this).

Someone pointed out how an attempt was made to teach a child Klingon, and
that it ran into trouble because Klingon didn't have enough vocabulary.  I
envision inventing words as necessary, in essence inviting the child to
become a participant in language creation as it is being used.

The child points at something: "What's that?"
"I don't know.  What do you think it is?"
At that point, the child could invent a word for it.

Comments? Questions?

A related idea is: how can we distill grammar to the simplest possible
state, and likewise with phonology, while preserving a usable language?  I'm
inspired by Virginia Keys' language Cho Ron that seems to get away with
eliminating verbs.