I have seen Dairy show cattle fed dried beet pulp to prepare for showing. I don't know how young you can start them, it would depend on their rumen development and I suspect it's still way too early. Increased intake of a high quality texterized 18% calf starter would be best. Late March means they are only about 4 weeks old now, although expensive a 3rd daily feeding of milk replacer would work. Sometimes sprinkling a bit of dry milk replacer on top of their starter feed will increase their desire to eat more starter. As their intake of starter increases they'll take off, it's still very early and soon hopefully you'll be saying... hey guys slow down you're eating me out of house and home.
By the way.... :welcome: to the boards.