As-Salamu alaykum sister,
While I am not an Islamic Scholar, I do suggest encouraging her with positive reinforcement, kind words, and gentle encouragement. It is difficult at this age and we must guide with patience and love insha’Allah. While she is 12, I am not sure of her maturity level as each child is different, however 12-13 is about the age when children begin fasting. I kindly suggest that when she gets tired, divert her attention with something fun or invigorating that will take her attention off of her fatigue and place it on another interest. You may have to do this many times to condition her mind and body and strengthen her resolve. After Iftar, tell her how proud of her you are and how pleased Allah SWT is with her efforts. Even if she has failed. By reinforcing the positive it will give her a sense of accomplishment and insha’Allah will inspire her to try harder. Pray with your daughter and make duaa to Allah that she becomes use to fasting and becomes able to complete it. Allah is most merciful.
Your in our prayers sister please let us know how you are doing.