How to remove an object's children?

Iterate through the table that GetChildren gives you and delete each instance. Destroy cant use the information in a table, only on single instances. Like so:

local Children = player.Backpack:GetChildren()

for i=1,#Children do
	Children[i]:Destroy()
end

Now this is quite complicated so let me explain. First you store the table of children in a variable so it can be properly read later in the script.

Next create a for loop that executes once for every child in the backpack. This is done by using #, an operator that counts how many items are in a table that it is in front of, so now it will go up from the first number (1) to whatever number follows it (However many items are in the table). i (Or whatever variable name you want, it doesn't need to be the letter i) can be read later in the script and it will tell you which loop it is currently on.

Next destroy each child, this is done by getting each item in the table and destroying it. This is coded by putting a number at the end of the table with square brackets around it, if you use i instead of a number it sorts itself out.