There are several tricky grammatical things here...
1) This is the one that actually answers your question. The key here is the word "dem" "dem" is the dative case version of "the" for both masculine and neuter nouns, but only in the singular. The dative case version of "the" for all plural nouns is "den".
2) You may be wondering why "Jungen" is in the dative case here at all? It's because the verb "reichen" always take a dative object. There's quite a few verbs like this. The best list I've been able to find online is the one here:
3) The noun "Junge" is a member of a rather large group of nouns called masculine "weak nouns" or "N-nouns", and their odd characteristic is that they take an -n or -en ending in every grammatical case except nominative. That is, if they aren't the subject of the sentence, then they end with -n or -en... even when they're singular (but also when they're plural).
For "Junge" specifically, this can be seen in the table here: (sorry for the long link)
For more information on masculine weak nouns in general, see:
Thank you for that information. Is the key whether the object of the verb can be defined as an indirect object (e.g., "geben ihm" is to give something to him, versus "geben ihn"--give him [to someone else]? I'm not sure it's obvious in this case, but if the guidelines works it might be in theres somewhere?
Technically, it should be OK, but it's awkward in English. "Suffices" can be used as a transitive verb, but it's more often used as an intransitive, in which case it would have to be ". . . suffices for the boys."
In addition, "suffices" isn't used that much in informal English--it sounds more natural to say ". . . is enough for . . . "