What are the odds of passing down IVs without a destiny knot?

Are they completely random or is there a chance they'll be the same as the parents.

I believe the child will inherit 3 IVs from its parents, each from a different stat and from a random parent. Destiny Knot simply changes this from 3 to 5 (but that is very significant).

You can also use the Power items (Power Anklet, Power Bracer, Power Belt, etc.) to force the parent holding it to pass their corresponding stat's IV to the child. This wont increase the amount of IVs passed i.e. if the other parent is holding the Destiny Knot you'll still only get 5 IVs passed down, 3 otherwise.

What I'm trying to do, I have a perfect IV Dratini from wondertrade but its name is 129 so I want to breed another. How hard will it be to get the 6th stat perfect?

Without a destiny knot, even with two perfect parents, the child has an abysmal 1 in 32768 chance of being perfect as well. With two perfect parents and the destiny knot, the odds are just 1 in 32, much more managable.

If all you have is the one perfect dratini, odds are still good the first few children will have 2-3 perfect IVs... then you swap them in to breed with the perfect parent and the next generation of children will probably have 4-5 perfect IVs. Then with the 6 IV parent and a 5 IV parent, odds are about 1 in 55, very close to a 2% chance of a perfect child. Wouldn't take long at all tbh. Keep in mind if you exclusively use either physical or special attacks on your Dratini that '5 IVs' is perfect instead and would be much faster to acquire the desired 31s you want.

Don't forget to everstone whatever parent has the desired nature so you keep passing that down too. The perfect IV parent you got from wondertrade doesn't need to have the destiny knot attached to pass down its IVs - either parent can hold the knot and it will have its effect on the resulting child.