No "see"food

Ok, so lately I have been trying to help Jes and Wilk understand that they need to keep their mouths closed while eating. Unfortunately, that is easier said than done. They literally cannot seem to keep their lips shut while the inside of their mouth moves. This is totally baffeling to me. Asher even shows them how to do it. But the more we show them, the faster they chew with their mouth open. We have not been able to make them understand that they need to keep their lips shut but can keep the inside of their mouth open so they can chew the food with their teeth instead of letting it dissolve. Wilkenson tries really hard. He has started holding his lips shut with his fingers and when all the food in his mouth is gone, he will say very proudly with his arms up high, "TA DA!!!"