You can try mixing their crumble with warm water to medium oatmeal consistency and hand feeding it to them while they are snuggled in your lap.
They are fussing when you walk away because they see you as Mom and want to be with you. Glad you separated them from the chicks. That must have been scary.
Now for the tantrums, they look at you as Mom, so treat the tantrums the same. Ignore it. Put their feed out and fresh water to wash it down with. Talk to them and play with them, but don't cave on the lettuce. Romaine is ok, but remember, if you have gardens with lettuce growing, your geese have learned to like it and will destroy your gardens to get to it and will discover other yummies you have there along the way. Get them used to grass and weeds (mine LOVE dandelions) and they will be great at weed control. Once they are grown, if you have enough forage for them, the feed they will consume will be minimal. But they need the nutrition now that feed gives them. Treats can come later.