I live in Harpenden. I moved here in 2006 as my husband has family here.
It is a popular place to raise children and there are regular trains into the city on the Bedford to Brighton Line.
I have a 13 year old son who goes to one of the Harpenden State schools, which has an outstanding reputation. All the schools in Harpenden are excellent and you pay the price for this by way of a property premium. House prices in Harpenden are high.
There are nice villages outside Harpenden but to guarantee a place at one of the schools you need to live in Harpenden itself.
There are lots of nurserys and i am sure there are toddler groups. I am pregnant at the moment and so don't yet know myself about what is on offer for young children and their parents but there are so many pregnant women and women with pushchairs in Harpenden that there are bound to be plenty of groups, at least i hope so. My next door neighbours moved to Harpenden after us and didn't know anyone and they have a network of friends through their kids.
Harpenden is very green, with a lot of common land and Rothampstead Park has a children's playground as does Batford. There are also several offroad cycle/walking tracks.
Regards to areas, there are not really any bad areas. If you want a character property that is within walking distance of the station and close to a secondary school and on a quiet street, you will pay more than you would for a newish build property on the outskirts. The closer you are to the High Street/Station the more expensive it gets.
Let me know if you would like to know anything else.