Zirtek syrup, otherwise known as Benadryl. I'm not sure what the label says re age limits, but dd2 was told to take it by the GP when she was 2.5 - did'nt really need it much last year, but now (3.5) she has it every morning and it does the trick.
I only use Piriton if she is very runny/sneezy at night, because its sedative.
HTH