One way to teach simply is to use flash cards or a chart. While a long sentence definition may be out of reach, three or four words may be grasped. It may clarify definitions for the child. Parents can supply sufficient details to make the basic definition understandable. In other areas of the catechism, the Ten Commandments, for example, stories are useful, to illustrate each Commandment expressed in a very few words.
Many children can learn, if taught.
Remember teaching does not require an immediate response, like testing does. Continue instruction by taking the child to church to Holy Mass, and a visit to the Blessed Sacrament. Say the Stations of the Cross. Light a candle. And pray for priests and bishops, that they provide for the reception of the sacraments by disabled children.