I also think that the problem here is the Reception place.
Places in all years do crop up in excellent schools on a regular basis - people move, decide to go private etc. So my feeling is that you'd almost certainly get a Year 2 place somewhere in a good school in St Albans or Harpenden - but the problem is that reception places are at a premium in both towns. There are now many families left without a reception place at a nearby school every year in St Albans and there are several campaigns to build a new school.
However, if you get a place for your Y2 DD, then your reception age DS automatically goes to the top of the reception waiting list as a sibling, iykwim?
Independent school may not necessarily be an easy solution etiher because most are single sex. The popular choices in St Albans are St Albans High School For Girls (v academic and competetive entry, mind) with its junior school in Wheathampstead; Abbots Hill (girls) - less academic, near Hemel; St Columbas (less academic) for boys in St Albans. The only mixed prep I can think of is Beechwood Park, near Markyate.
If moving to St A, the best secondaries are on the north side of the city. Also very much oversubscribed too I'm afraid!
You might look at the St Albans local board as there are several threads there asking v similar questions to you