It is a difficult expression to translate with uncertainty regarding its exact meaning. What is certain, is that it does not refer to the enemies mentioned in the previous verse, because the crying is to God for help in distress. The following possibilities as to whom are being referred to have been proposed:
The inhabitants of Jerusalem, for they are crying to God. According to the first sentence of the verse, it may be that they are already crying out and are further summoned to more and better praying (see the rest of the verse).
It is also proposed that the hearts of the children are crying out and now their parents are summoned to pray for them. The description of the children’s suffering in Lamentations 2:11–12 and the mentioning of intercession for them in Lamentations 2:19 supports this idea.1
18 Their heart cried to the Lord. O wall of the daughter of Zion, let tears stream down like a torrent day and night! Give yourself no rest, your eyes no respite!