A one amp supply will be enough or the D cells will be replaced very often.
You can measure the current required with a meter. Put the batteries in but
don't allow one contact to mate with what it wants to touch. Put a piece of paper in the way. Now use the meter, on the 2 amp range, for instance, to complete the connection the paper is stopping. The meter is now in series with the power supply and can tell you how much current is being used. Sellect an adapter 40% bigger.
I would just use a regulated 1.5 amp universal adapter. This way it can be used for any purpose in the future.
Is this device to be used outdoors or near water? Take very good care that the supply is protected from the elements. If there is a chance that 110v or 220v will find a leakage path to the "swing" then it's not safe.
For outdoor use, you might use a 6v auto or motorcycle battery and fuse.
Just recharge it as required.
Oh yes, your other question.
Use a "switched power jack" (single pole, double throw) on the swing.
When you plug the adapter in, one (of two) connection to the batteries is broken and the one from the adapter is "made".
Break one connection from the batteries to the swing.
Connect this to the power jack (the arrow one in the pic).
Connect the "common" connector on the jack to the batteries wire that you have not messed with.
Connect the switch "male" pin to where the battery wire used to go (the swing).
Be sure the voltage polarity on the switch jack/adapter is correct!
