It's out of order. I'll indicate why, and then I suppose you can appeal a decision of the chair, if you wish.
Clause 6 of Bill C-44 defines a child as a person who is under 18 years of age. Amendment NDP-4 proposes to extend this definition of child to persons over 18 years of age. As House of Commons Procedure and Practice, second edition, states on page 766:
An amendment to a bill that was referred to a committee after second reading is out of order if it is beyond the scope and principle of the bill.
In the opinion of the chair, the amendment NDP-4, which enlarges the definition of “child” to persons over the age of 18, is beyond the scope of Bill C-44 and is therefore inadmissible. At this point you can, if you wish, appeal the chair's decision or challenge the chair.