बच्चा=child ( male)
बच्चे= children ( mixed group of male and female kids OR all males). ( even if one child is Male in the group then too it will be बच्चे )।
बच्ची= child ( female).
बच्चियां= children ( all female).
Why baatchi and not baatcha ? (Sorry for the spelling)
बच्चा is a male child and बच्ची is a female child. Both are acceptable here.
If you use बच्चा, make sure to conjugate the verb as masculine - वह बच्चा जा रहा है
Also, there is no 't' sound in बच्चा/बच्ची. Instead, the च sound is stressed.