The first assignment for the trombone is to properly remember each slide position.
It's not as simple as pressing which piston or which key to press like other instruments, but the only instruction you can give is "roughly around here" for the slide position. The 3rd and 4th Positions have markers in front of the bell and on the other side, but there is not even that for the 2nd and 5th positions.
The best way to properly remember the positions is to play the following sound patterns with a euphonium or something like that while checking the pitch.
The last 7th position is difficult for elementary school students to reach, so practice up to the 6th positon.