A suggestion for an exercise:
Select a short passage of music of say four bars that involve everyone and has room for some conducting work in terms of dynamic or tempo changes. Go through it until everyone can play it without looking at the music. Then have everyone look at you, while playing that passage over and over again. Each time you do something different through your conducting;
have them all play loud,
have them all play really soft,
have some play loud and the rest soft,
do a crescendo and/or a diminuendo,
do a subito pianissimo,
make them emphasize one specific note,
have them play very legato,
have them play bouncy, joyful, sad...,
pull the tempo up,
slow the tempo down,
and any other stuff that you can think of that makes looking at the conductor relevant and the exercise fun.
This will hopefully practice the musicians in following you when they actually are looking, as well as teach them the importance of looking at the conductor.
When you are done with the above, start some bars ahead of, and carry on for a few bars after these exercise bars, and tell them to beware (look up!) when you reach the exercise bars where you will do something of the above. This will hopefully practice the musicians in looking up from the music, and then to look down finding the place in the music again.
Another suggestion for an exercise:
Play a piece you find fit for this exercise; After every two bars (or any regular suitable low(!) number of bars) everyone stops playing (perhaps while counting the beat out loud) and looks at you for a full bar before looking down (if necessary) and continue playing at the start of the bar after the last played bar. During the counting and looking "intermission" do some dynamic or other conducting instruction as suggested in the other exercise above and have them stick to this when resuming playing until the next "intermission" where a new instruction sets in.
This will hopefully also practice the musicians in looking up and down in the middle of the music.
This exercise might be hard, so start very easy. And be sure to find a suitable easy piece of music that doesn't have so many notes that are held over consecutive bars. (Instruct the musicians to break those notes in two so that they can end and start exactly at the bar ends and starts.)
When you are done practicing the above, play the same piece again and have them look up (for a short glance at you/the conductor) at the same places but keep playing without "intermission". Hopefully you will have opened(!) their eyes by now.
Yet another suggestion for an exercise:
Play a piece from the beginning (or some other good place to start). Before this, inform the musicians that you will suddenly signal to stop on the first beat of some random bar, and that the goal is for everyone to stop collectively and to have complete silence after this stop beat. Repeat this several times. Start by stopping after the first or second bar! Then, for each restart, increase the number of bars you go before signalling to stop. Throw in an early stop once in a while. Finally play the whole piece through without stopping! The idea is to force everyone to give attention to what you do in order to not miss the stop signal.
You can increase the difficulty by allowing to stop at any beat.