While I agree with the opinion that as a 'stand alone' device the Fastrack gates/flashers are probably the best bet., for a 6-year old they can be...challenging..., especially if teardown and re-building of the layout is part of the play/creativity he/she will be doing.
The two 5" gapped 'trigger tracks'...our (LHS) term...sections that MUST be present, properly aligned, and oriented in order for the accessory to work at all, are a big part of that challenge. After initial sales years ago upon their first release resulted in many being brought in for a refund on the premise of "Not Working!", we instituted a policy of demonstrating at the counter for the customer...young AND old...at the time of purchase the proper arrangement of the three pieces for installation. We also reference the instruction folder pages that state this. To no one's amazement, many of those 'not working' purchases made a 180-degree trip back out the store door after they were proven at the counter to work...after all. Sort of a polite "Duh!" moment, if you will.
Then there's the annual "How come?" phone calls....'How come the accessory worked last year, but when I set it up under the tree this year it won't work, or works all the time never stopping, etc., etc., blah, blah.??" The polite verbal 'Dinozzo' reminds them of the 3-piece idiosyncrasy of the accessory. Or the search for the elusive gapped trigger tracks begins. Or the gapped trigger track having been placed elsewhere on the layout resulting in weirdness of train operation.......all part of the seasonal 'fun'!
That said, the gates are another matter. They don't like manual encouragement!...at all! I'm told the latest version of the accessory is more forgiving of this element of 'play', but this retailer's jury is still out on that claim.
The commentary regarding the co-axial plug attachment of the crossbucks on the flashers-only accessory is spot-on. This past couple weeks we've tried to analyze the seeming incompatibility of a pair of accessory bases and an assortment of flashers as to the cause of inconsistent performance....to no avail.
And neither accessory...gates/flashers or flashers, only...are tolerant of accidental sidewards force....e.g., derailing speeding trains, swinging arms and hands, kicks of the feet (floor layouts), and curious swipes of the cat's/dog's paw....among the reports. Yes, they're very transportable and realistic. No...with caveats...they're not as robust to play....especially rough play....as the main premise of Lionel advertising would encourage.
As for the "expensive" part?......shop around. There is a variety of street and cyber prices out there. Beware the garage-sale deal, though. This accessory...unless for static display, only....is definitely one that needs all THREE pieces to operate, and should be demonstrated operable before buying!!
And, yet, as to the original request, it is, indeed, an excellent choice.
But, as always, that's just MHO/OHO.....FWIW.
KD