They soon get used to negotiating buses etc and it's a useful life skill to develop
. If you are going for the train/bus route, have a look for the Greater Anglia Scholars Season Tickets website. The other grammar schools in the Southend area subscribe to it so I expect WHSB does. You need to get a rail Photocard for your DS first, but then you can book termly or academic year season tickets at discounted child rates and add on a 'PlusBus' season (unfortunately not discounted) which saves having to remember the bus fare.
An alternative route is Romford-Upminster-Chalkwell (c2c), but it is a 20-25 minute walk to the school at this end, quite hilly to boot. Or Romford-Upminster-Southend Central, then 21 / 29 bus as before, but I doubt that too many do it that way.