This is part of the fanon universe of the 2012 TMNT series. It is the first episode season 1 and the episode plot is extremely similar to the 1990 TMNT episode "Back to the Egg".
A strange gas is spreading around the city, and it somehow reverts the teenage turtles into two-year olds. How can April deal with wailing little mini mutants while preventing the gas' effects from making the TNMT so young they'll cease to exist!?