I entered У ребёнок нет мамы and it was incorrect. The correct answer given was У ребёнка нет мамы. I read the lesson notes but even after that I can't understand what grammar rule is causing this change to arrive at "ребёнка"
When using "у", the genitive case of the noun must be used. In this case, the genitive case of "ребёнок" would be "ребёнка".
Note the vowel disappearing in "-ок". This is fairly regular for this suffix.
Which might not be obvious for a non-native speaker ;)