How to pass parameters to callbacks in flutter?

You call widget.changePrice(childPrice); from OneItemState, so changePrice can't be VoidCallback

typedef IntCallback = Function(int num);

/* ... */

class OneItem extends StatefulWidget {
  OneItem(this.combo, this.index, this.changePrice);
  final Combo combo;
  final int index;
  final IntCallback changePrice;

  @override
  State<StatefulWidget> createState() => OneItemState();
}

/* ... */

ViewCard(args.combo, _changeprice) // function name without braces

I would do it like this

You could declare a Function callback, like this:

final void Function(double price) changePrice;

Here another example to understand it better:

final bool Function(String id, double price) updateItem;

bool isPriceUpdated = widget.updateItem("item1", 7.5);