The tense used in the above sentence is usually referred to as just the "Present tense", it is, in reality, the "present habitual". This means that it describes what one does on a regular basis. Therefore, the above sentence suggests that the boys read every day or every week, etc.
I found this http://www.gaeilge.org/ConjugatingRegularVerbs.pdf it's a guide on Irish conjugation in many forms, present tense included. The root is not lé, but apparently léigh, and it's a rule to drop the '-igh' and add the normal conjugation endings. They use 'leigh' as an example, but instead of adding the endings directly on, they seem to elimate the 'e', ending in a result of 'léann' rather than the 'léeaan' that would be expected from the aforementioned rule. I guess you just ignore the extra 'e'?
The notes don't explain the situation with léigh. It's first conjugation. In fact, you can see this from then -ann ending.
The main thing is that first conjugation verbs all have one syllable, while second conjugation verbs have two or more.
The notes say that if a second conjugation verb ends in -igh, drop the igh and add the second conjugation endings. A clearer phrasing would be as follows:
If a one-syllable verb ends in -igh, drop the igh and add the first conjugation endings; if a two or more syllable verb ends in -igh, drop the igh and add the second conjugation endings.
Léigh is one syllable (sounds a bit like lay), so just chop off the igh and stick on -ann.
A good overview: http://www.gaeilge.org/ConjugatingRegularVerbs.pdf
"Leann" is the present (habitual) of the verb "léigh" (to read). So it is "something I do regularly". So, "Léann siad" is they read (every day, every week, every month). To say "they are reading" you would say "tá siad ag léamh". This uses a form of the verb called the "verbal noun". It's best not to worry about it for now though :)