Don't use mealworms they aren't a good feeder. Get super worms instead, look similar but better nutrition. You can try silkworms, hornworms and dubia etc. Finding small ones will be the challenge.
You have try multiple times to get them used to something new. Mixing different feeders together so they get a new one by accident will work sometimes. I wouldn't do it with a youngster but you can withhold crickets for a day and then give the new feeder on the next day when they are hungrier.