As the father of a 2 year old and a 5 year old, here are my 2 cents.
We have the wooden trains (Thomas, Brio, etc.); Lionel's Thomas LionChief remote version and The Polar Express LionChief remote version. The wooden trains are great for their fine motor skills and creativity and no need for parent oversight. The Lionel products are great for their size, SOUNDS and SPEED!!! (Emphasis provided by the resident 5 year old).
Now, what gets played with and how...
1. Wooden trains: every so often once they are about 3 years old, funny thing is they always want more track...don't know where that comes from!! Easy to travel with. We have found that the wooden train table rarely gets used, 99% of the time the track plans are set up on the floor due to space. If they opt for the wooden trains skip the table and save the money. Get the table if you want to TRY to coral the set up and avoid late night foot pain from stepping on the wayward train cars.
2. Lionel remote sets: used every so often, most 2 to 2.5 year olds can use the remote with ease and enjoy making the horn/bell/whistle sounds blast away. Keep track plans without switches as they enjoy speed and running and blasting whistles. We use a lot of cross over setups. Needs adult set up help, but with the right training the kids understand how to treat the trains nicely. The Thomas set is very robust and even the littlest ones can't do too much damage. The Polar set is more prone to needing help after being kid handled.
3. MTH DCS: My oldest son was fairly proficient with the DCS remote at age 4. But that is a lot of money to invest early on for the kids and so much is changing right now (apps coming from all manufacturers). I'd vote for going this route 5-10 years down the road if they are still interested.
4. Lionel MPC locos and dummies get more work than all of the above being pushed around my layout and the kids' carpet layout. There is something the kids love about pushing the big trains in their hands and controlling the movement. See photo above posted by Aldovar for proof positive this is just as good an option as any!!
Enjoy!