<child-component #child></child-component>
<button (click)="child.anyChildMethod()">Click</button>

<child-component #child_0></child-component>
<child-component #child_1></child-component>
<child-component #child_2></child-component>

<!-- Pseudo code -->
<button (click)="'child_'+componentVariableWhichHoldsNumber.anyChildMethod()">Click</button>