Mexican officials said six children were wounded and another girl missing after the attack, which left the family's three cars riddled with bullets and utterly charred, as seen in images posted on social media by devastated relatives.
"If Mexico needs or requests help in cleaning out these monsters, the United States stands ready, willing & able to get involved and do the job quickly and effectively," Trump wrote in a tweet.
He praised Mexican President Andres Manuel Lopez Obrador for prioritizing the fight against drug trafficking, but said "the cartels have become so large and powerful that you sometimes need an army to defeat an army!"
"This is the time for Mexico, with the help of the United States, to wage WAR on the drug cartels and wipe them off the face of the earth. We merely await a call from your great new president!"
Lopez Obrador said he would call Trump and accept "all necessary cooperation."
But the left-wing populist, who has declared an end to Mexico's "drug wars," said he was not looking for a new one.