When completed, this sentence translates to: "It is essential that children learn about the dangers of the internet in a safe way."
The beginning of this sentence ("Es imprescindible que", meaning "It is essential that") should make you aware that it requires the use of the present subjunctive. The present subjunctive is used to talk about situations of doubt, desire, emotion, necessity or uncertainty. In this case, the sentence starter highlights that there is an element of necessity to the statement that is to follow.
To use the present subjunctive of the verb 'aprender' in the third person plural form (since the subject of the sentence is "los niños", plural children), you firstly take the present indicative form: aprenden. You then remove the '-en' ending and replace it with '-an': aprendan.