As inthename said it will be in year admissions. Our school year starts in September. When does your son turn 8 as that will affect which school year he goes into. If he is 8 before 31st August 2015 then he will be in year 3, or if after that he will be in year 2. Also remember in the UK children start school in Year R (Reception) the September after they turn 4, so dependent on your daughters birthday is she might be in Reception or Year 1.
Most of the primary schools in Horsham are very full, especially in years R to 2 as they are unable by law to have no more than 30 children per class. However in year 3 there can sometimes be a little bit more movement available.
With Horsham it does depend very much where you will be living and you certainly won't get a space allocated until you have the address you will be residing at.
Because of the pressure on school places in the town at the moment some people are being offered spaces in village schools surrounding the town, e.g. Warnham & St Andrews Nuthurst.
Your best bet is when you visit in September is to arrange to visit the local schools who might have spaces available or that you would like to go on the waiting lists for and discuss your sons needs.
The schools I would be looking at are: Kingslea Primary School, North Heath Primary School, Holbrook Primary School, All Saints Primary School and Arunside Primary School. Trafalgar Infant school is a possibility which then feeds into Greenway Academy in Year 3, (although mixed reports about Greenway at the moment), also Littlehaven Infants which feeds into North Holmes Junior School and lastly Leechpool Primary School.
If you want to pm me to find out more about their locations etc please do so & I will try to help. (I'm a childminder in the town whose 2 dd's are now almost 17 & 20)