means blablabla should be the way to go. in this case : Jesus told them, act like a child should be the way to go. 做好榜样才行。behave good should be the way to go. 做完作业才行。finish your homework should be the way to go.

才行means it is okay. it is good. as you said 耶稣告诉这些大人，他们的态度要变得像小孩一样才行 this 才行 is to make a compliment of the previous sentence. to confirm the sentence as a 'good way'