I run N gauge stuff and there’s a 5 year old from next door who routinely destroys it when he pops round- I don’t mind but if you were giving it to a five year old it wouldn’t last two minutes.
My family compromised with Tomy stuff when I was little. It’s not mad cheap, but you can build up something good and tough for not a lot.
http://www.ebay.co.uk/itm/Tomy-Tomica-Super-KOMACHI-Shinkansen-Train-Set-With-Extending-Magnet-New/141134426310?_trksid=p2054897.c100204.m3164&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140407115239%26meid%3D9d142f329d074875bbee4378604566a2%26pid%3D100204%26prg%3D20140407115239%26rk%3D3%26rkt%3D30%26sd%3D141139802397
If you’re set on a Hornby style set up then Ebay and Gumtree are good places, particularly Gumtree if you want to pay less money for it.
Points are quite expensive, even secondhand, so think carefully about how many you need. Second hand straight track is cheap as chips, though (I think I’ve got some if you e-mail me, and a few other bits). As above, flexi-track is very cheap- metre lengths are £3 or so and don’t have the problems of dodgy electrical connections every 6 inches.
Find a decent tank engine (tender ones are more fragile) from Hornby’s railroad range (I like the SDJR Jinty -http://www.duncanstoychest.co.uk/hornby-r2882-00-gauge-s-and-djr-0-6-0-3f-locomotive-railroad-locomotive) and search “rake” on ebay and you’ll find sets of secondhand wagons and carriages going cheaply. I reckon you could get a new tank engine (second hand from ebay can give you a dodgy runner which will frustrate him) and a set of wagons for £50, then a load of flexi track and three points for about another £40, leaving you a tenner for a Hornby R965 controller and you’ll be away!
E-mail me if you want me to dig out this track and stuff.
I know njee is on RMweb but if you’re totally new to it newrailwaymodellers.co.uk is less hardcore and very patient with what to them seem like very simple questions.