hi charli
i am living in bali since 2 years with my 3 year old.
my area is petitenget ( seminyak)and kerobokan, choosen because its close from my working place.
sanur in the south has a nice beach( where children can swim and you can walk alsongside the beach, streets have sidewalks) and a nice and children frienldy expat comunity.
also house rental costs are still affordable there which i can't tell the same for my areal here.....
Seminyak and Petitenget is more busy though, more restaurants, tourists and lifestyle venues.
some say sanur is little bit boring but i don't think so.
i think it depends what you prefer, there are some areas around semicyak and kerobokan which are lovely too( umalas for example( expensive) OR CANGGU( affordable) but the beach is not that good for children.
recommend you get a nanny and a housemaid to support you,both approx. 100 us a monthe each, nanny more if you want a real good one, i got mine from bensbali, a agency.
good luck and all the best, maybe see you around some kids spots.