ok first off...the trex diet has every thing they need, an awesome product! Don't add any supplements.Too much D3 can be bad...the diet is formulated perfectly.
To get them onto the food try mix with banana(their favorite) baby food and leave it in the cage, put it in a small bottle top or what ever. Just stop giving crickets and monitor their weight. Gradually lessen the ratio of the banana baby food. It helps to put the "liquid" diet against a wall or branch so they can climb head first to it.
good luck! The trex diet is the healthiest way to go for your cresteds
oh and welcome! never seen yo on here before!