It is my belief that the ability to recognize distance from hand (slide brace) to mouth (mouthpiece) is one of the most natural instincts in human beings.
I am a trombonist myself and went thru music school and taught high school band and private lessons for many years. At a point in my career I chose to spend more time with family and took an all elementary band position. During this time I was watching my children grow and these are some of the things I discovered.
One of the first things infants practice, when they learn that they have hands, is that they can put things in their mouth. (This was VERY apparent after watching our toddler grab a "present" the dog had left on the sidewalk and try to put it in his mouth - I am glad we were fast : although this happened with amazing quickness).
The next thing I noticed was that our toddlers could watch TV, play with toys etc. while grabbing Cheerios off of the table and put them in their mouth, all the while not even looking at them. While you are watching the football game on TV don't you do this yourself with the popcorn?
At this same time I was currently teaching a group of beginning trombones that played well but stared at their slides all the time and thus were learning their music by rote. In other words, staring at the slide was interfering with the ability to see the music and maintain their visual tracking. (This was an extreme embarrassment to me since I was a trombonist myself). It was a real revelation let me tell you.
It was the combination of the discoveries above that changed my teaching of trombone. In the first lesson with students as young as 4th grade I now ask students to "feel" where the slide is while I place it in 4th position. We do this several times and then I ask them to close their eyes. The majority of the class can find this exact position with their eyes closed in the first lesson. We establish all positions this way maintaining 1st position as the home position (because it is closest to the mouth - remember that natural instinct?). As each position is learned we progress to the next using this same process. In my view, young children learn positions the easiest using 1st as home and beginning with the larger moves. (I usually use 1st - 4th - 6th). This alleviates the added difficulty of working on the limberness of the wrist needed to play in half step increments such as moving from 3rd to 4th. We get to that a few lessons later. We can get 3 positions established right away 1st - all the way up, 4th - just past the bell, and 6th - all the way out (most young students cannot reach 6th much less 7th - we just don't worry about it - "all the way out" is 6th).
While the purist will argue that each child should be taught by listening to the correct pitch to determine the slide placement, and although I agree that a great deal of emphasis should be placed on listening, the reality is that a young student's recognition of slight pitch variations is usually very limited and will be developed during their musical study. Pitch varitations are also dependent on embouchure fluctuations as well and these can happen quite regularly until the embouchure is sufficiently developed. In reality correct positioning is at least as important (if not more) in the initial stages. My efforts here are to encourage people to use the natural abilities of the students to help them maintain these positions.
Another common problem is that students will play with a stiff arm. Although this seems to work for widely separated positions at slow tempos the "lever" of an arm is usually too much for students to stop the slide easily in the right position especially in half step increments (adjacent positions). Usually a slight flexing of the wrist is needed in combination with some arm motion.
Students will also have a tendency to want to average their positions. If they move quickly from 1st to 2nd and then to 4th their brain will often want to average these distances into equal increments . Thus you might get a movement like : 1st - 2nd position out too far - and then 4th. Three equal moves instead of one short and then a longer move. Many many students play a flat 2nd position because they average and/or they don't know how to use the wrist. The most important use of the wrist happens when the slide is farther out (3rd to 7th position) because the "lever" or arm is longer and thus small arm motions have a much greater effect on how far the slide is moving.
It is a mistake to base the teaching of trombone to beginners based upon how hard it is for you (an adult) to learn. We sometimes make presumptions that have no basis when applied to beginners. You best learn how to adjust your teaching by clearing any preconceived problems from your mind and just watching what happens around you. If my one year old child could teach me lessons on the trombone perhaps my cat can teach us how to get bagpipers to play in tune.