5yo had middle ear fluid & adenoid hypertrophy, was on singulair (montelukast) 1 daily x3 months after which she developed sleepmyoclonus; can singulair (montelukast) b the cause?

Not likely: No Singulair (montelukast) is not likely 2 cause sleep myoclonus. The other conditions you mention are common in young children, middle ear fluid and adenoidal hypertrophy. Benign sleep myoclonus is also common, and unrelated to the former conditions. As long as your child's development is normal and the nature of the movement does not change, this is a variant of normal and needs no further investigation