You need to pick a set of syllables and stick to it.

My opinion is that the syllable doesn't matter so much as your ability to be consistent and INSIST that your kids are doing it the same. This starts in sixth grade.

Muddy/unclear can also mean bad tone quality, tuning, balance, and note endings just as much as note starts. These things are all connected.

Also, equally important to your articulation syllable is having air right behind the tongue. If airspeed is slow it doesn't much matter what your tongue is up to.