Hi been33, Lewes is brilliant and a great place to bring up kids. You will find most people really friendly. A few ideas to meet people/get out and about are as follows:
Sunny days head to Southover Grage gardens, where you will find most of Lewes mums and babies and then kids after school closing time, it is 5mins from the station
Kings church playgroup is meant to be the best Friday am playgroup but there is waiting list,so get your name down.
All Saints center in central Lewes has a playgroup on a Friday as well,a toy library on a Wedesday I think and the brilliant Tinklers music group on Tuesdays.
The library (next to the All Saints Centre) has music and story time things on once a month and a good kids area.
NCT is very active in Lewes and hosts a great nearly new sale twice a year and mums night monthly.
On the outskirts of town is Monkey Business, indoor play warehouse.
Pells Pool(outdoor swimming pool) has Monday morning under 5 paddlers group or go any time on a hot day.
There is a also a great free group called Inside Outside that meet weekly.
Much more as well but that is off the top of my head. Google anything and it should come up or feel free to ask me.
You will love it, great shops, cafes, mums, schools and views!