From what I understand about the OP's original concern, he wants kids under 5 to be able to run the train around the Christmas tree. I am not a fan of LionChief for my own operations but I have several Thomas and Friends engines that are nearly bullet proof for kids, even for my 2 year old granddaughter. They have young kids versions of the LionChief remotes that can't be beat in my opinion. I also have a Silver Bells set that is LionChief. The kids can run that just as well.
I hate to say this but even an electronics upgrade wouldn't "simplify" operation. And IMHO, you need to keep young children away from the transformer. LionChief both simplifies operation with only a throttle and 3 buttons for sound and it keeps young kids away from the electricity.
The thing I don't like about the LionChief stuff is the power they come with. The wall wart provides ludicrous speed. When I run them on my regular layout, I turn the ZW down to 12 to 14 volts to keep Thomas from going crazy.
To the OP, if the recommendations here are not satisfying your concerns, please elaborate on exactly what you are looking for. I agree with another post here that we are recommending you replace the engine without knowing exactly what you are looking for. I read "simplification of operation" and I believe LionChief is the simplest thing going.
Tony