There isn't much to be said about this movie. All it really is is a cash-grab, direct-to-video fodder and the creators declaring creative bankruptcy.
The plot centers around the main six kids getting captured by the kindergartners. How do you make up for the blatant lack of action following the quick capture? You do the most pointless, non-creative trope known to man. Flashbacks. I personally hate this because it'd give the writers an excuse to not create more animation and instead pad out the running time with already completed animation.
With this move, they show episodes from the earlier seasons, and the animation clashes dramatically with the new animation in the movie. The worst part? They just show the full episodes that correlate with each flashback. Totally creative, amirite?
But the straw that broke the camel's back was the final flashback, one that disregards what was previously established in one of the earliest episodes in the series. It essentially serves as a power trip for one of the characters, Gus. Ever wanted to know why the kindergartners act like some primitive tribe? We have Gus to thank for that. Even better, they make it so it seems like it actually happened. May I ask why?
Aside from it also being a pretty close reversal of the Rugrats special All Growed Up, there really isn't much else to say about this movie. Except that it's a huge waste of time.