The kids ran the bases at Citi Field on Sunday after the Mets’ dreadful 2-0 loss to the Cubs.

Even they could have scored on the Mets’ lack of anticipatory defense.

Every game is a new learning experience for Mickey Callaway when he discovers just how much his Mets don’t know about baseball, and Sunday was a particularly painful lesson.

Once again it is back to the drawing board for the Mets, and at this rate soon it will be back-up-the-truck time.

Steven Matz was terrific pitching to batters, but allowed Javier Baez to steal home on a double steal in the seventh inning with a lollipop throw to first base, where Willson Contreras baited him into that throw.

Later in the inning, Contreras, the Cubs’ catcher who caught all 14 innings Saturday night, used his speed to score on a pop fly to short right, 195 feet from home plate, because right-fielder Jay Bruce did not call off drifting second baseman Luis Guillorme.

It was another fundamental mistake in a long line of mistakes the Mets have made this season.

Defense has never been the top priority with general manager Sandy Alderson and his home run-based offense, and the Mets continue to pay the price for that style of play.

Defense matters. Especially when you can’t hit.

The Mets have scored one run in their past 24 innings, dating to the ninth inning of Friday night’s 7-4 loss to the Cubs. Joe Maddon’s team ran the Mets ragged, winning all four games as the Mets lost for the ninth time in 11 games and have lost six straight at home, where they are 12-17 this season.

At this stage, Callaway could call a team meeting three times a week to go over the Mets’ lack of fundamentals.

Callaway & Co. have to find a way to clean up this mess.

“It’s frustrating,’’ Callaway admitted. “You’re giving them the game instead of them winning the game from you. And that’s hard. If you do things the right way and you get beat, you don’t feel great about it because you lost, but you can accept it and move on. We have to learn from those mistakes and make sure they don’t happen again.’’

The Mets’ lack of defensive awareness at times is staggering, and the word is out that the Mets are such a poor defensive team. Other teams are beginning to take more advantage.

“When you are an outfielder, you have priority over the infielder,’’ Callaway said of the pop-up RBI by Ben Zobrist.

That is Baseball 101, but Bruce gave way.

“I think we need to shift our focus,’’ Callaway said. “We are not focusing on that part of the game very well. If we have to go out and work on cutoffs and relays and pop-ups and PFP [pitcher’s fielding practice], that is what we will do instead of being on the field and hitting. I guess we need to make some adjustments on what we are focusing on.

“We go out there and do defensive work every day and we go out there and hit, maybe we need to shift our focus to make it more stuff like just ground balls and throwing to bags to be more fundamentals specific,’’ Callaway added.

“I think it’s focus.’’

Callaway is onto something: The Mets need to improve their situational focus in games.

“When you are out there, you have to be thinking of every situation that could pop up so you are prepared to go do that,’’ Callaway said. “When guys are struggling, their mind may not be in that place. Maybe they are thinking about an at-bat they just had instead of in that moment focus on what could happen.’’

Matz was spectacular, except for that pick-off throw. Over the last 16 games, Mets starters own a 2.48 ERA, yet the Mets are 5-11 in that span.

All their issues are coming to the surface.

The Mets are in a terrible spiral and at least have to fix what is fundamentally wrong.