Is. 11:6 represents a New, or the next earth-age where carnivores no longer are meat-eaters: the big bad wolf will live with the lamb, and a little child shall lead them. A quick look around reveals, that this is not, that time.
Mark 10:15 is speaking in the current: in order to enter into the Kingdom of Heaven, on earth required the vision, faith, and humbleness, of a child. The Scripture was True - when spoken by Christ Jesus then, and now.
Matt.18:1-6 is additional witnesses and details of what is required as adults, to enter the Kingdom. "Except ye be converted [Repentance], and become as little children, ye shall not enter into the Kingdom of Heaven".
People can not see the Kingdom with worldly eyes, hears that refuse to listen, and a mind that is hard. Children desire to believe; and have no problem with humbleness. Especially during these times [Fig-tree-generation] we should pay extra attention to children - as an example.
Isaiah 3:1-12 = bad judgment. And children play a role in this as well. These are immature adults with all of the characteristics of teenagers. This is the time of "sorrow" [labor pains] spoken of by Christ Jesus. The trouble [test] will continue to get worse. Take a look at Centralized-government!