Dear April,
I have to agree with you - accuracy counts. Your style and technique ( BTW I feel your pain, as I wish I could paint more loosely, too) will just emerge and become more distinctive over time. There are several components to accuracy, and drawing is key. Check the size and placement of the eyes and the relative width of the nose. I would also look at the overall shape of the head, and how the hair sits on it.
Your reference photo has beautiful lighting - 18 months is a very difficult age to photograph. You made a good decision not to paint his right arm heading out of the frame. The square-on shoulders and face present a design problem that is virtually impossible (at least for me) to solve. Still, those 18 month-olds, they just won't follow instructions
You've done a nice job keeping your edges soft, which sets you up to make just one or two shar edges and really gain visual impact.
I think that your painting would benefit from some light values - the uniformity of the middle values tends to reduce the sense of form. I am not sure what your experience with the wipe-out method is, but once the paint sets up to a certain point, it's impossible to get back to the lights - perhaps that worked against your establishing a larger range of values. Th value in the whites of the the shadowed eye are a bit too light and make that eye a bit flat .As far as the distribution of values, the upper right and lower left areas are quite similar in shape and size, so more variety might give a sense of movement to the piece.
Is this the full painting? If so, the head would be much larger than life size, something which can make it harder to see drawing distortion.
I think that you are lucky to have this little model next door- you defintiely have the lighting inhand!
Good luck, hope this helps.