Hi,
I'm new to this forum. I have a bit expierence in adruino so don't hate me for maybe a stupid question 
Like the title says, I need a motor that pulls up to 10 kg for maybe 20 cm.
My plan is to build my own motor for a "swing 2 sleep" hammock. without motor it costs round about 100 € with motor 450 € ("Re:Furbed wie neu" Komplett-Set - heia 1.5 Motor, dream2sleep und Gestell – swing2sleep)...so at least I want to try it with my own build setup.
Any tips which motor is useful for that?
With a baby as the target I'd be inclined to minimise the DIY.
Scheibenwischermotor
I have seen them working with a simple solenoid. The plunger was on a circular piece of metal and as the swing moved back and forth the coil would energize when the plunger came close and turn off as it exited, repeating in the opposite direction. Wit this setup it is possible to operate it on a battery.
Use a geared motor driving a crank and something like a spring to drive the hammock ( so the motor is not stalled at startup )
The setup will have a natural frequency and drive at that rate .
As always the mechanical aspects are hard , electronics simple - probably don’t need a processor .
Look at commercial products to see how they work
A lot depends on the speed you'll want to pull upwards at. If this is slow enough a motor with insufficient torque can always be geared down (3d printed planetary gears are quite efficient).
If you are pulling and lowering with a pulley then the radius of it's winding drum is important too: narrower drum, smaller effective "lever", lower torque required, but slower.
For bouncing a hammock I'd ensure the mechanical design is such that if the motor fails the pulley can only unwind to a certain extent (lowed maybe by some sort of 3d printed friction brake?) before stoppng, not letting a hammock end drop unrestrained to the floor.
There are also such things as linear actuators, available and rated for plenty of linear force indeed. This sort of thing could, supposedly, raise and lower up to 600kg, slowly.
Be careful not to install that sort of option where it could crush anything when running in either direction.
P.S. as far as "mechanical aspects are hard" is concerned, this is generally true. If the mechanical can be done with the strength available in printed plastic parts then a 3d printer is the way to make mechanical parts feasible, I'm not sure anything else can give much hope of being able to construct designed mechanical brackets for anything. Without use of a printer I can't see how to do a good job of even using a linear actuator version. If you haven't one already, use the 350 euro savings to get a 3d printer, Creality CR20 pro is a half decent model, ender 3 compatible for replacement spares, but comes fully assembled. Pretty much any worthwhile project one might wish to build that involves a motor, needs the kind of mechanical parts a 3d printer can produce.