What do you mean by “loll, slop and lounge”?

• ‘loll’ means ‘to sit, lie, or stand in a lazy, relaxed way’.
• ‘slop’ here means ‘to sit in an untidy and disorganised manner’.
• ‘lounge’ means ‘to lie, sit, or stand in a relaxed or lazy way’. 

So, you see, basically the three words mean almost the same, i.e., to sit or lie in a lazy and casual manner, as the children do in front of a television set, as the poet has observed.

Here the poet meant to say that the television completely hypnotizes the children and children sit in a lazy, casual and relaxed manner and gape at the television screen all the time and becomes completely unaware about their surrounding activities.