There aren't feeder nurseries as such, but there are nurseries that have good connections. The closest to a feeder is miss Mynors in notting hill where big majority go on to wetherby and pembridge (but may be facet of everyone registered at birth). But that's A long way for your dss to go at 3.
Ours went to miss Delaney's in holland park, good 15 min walk or scoot. You should register ASAP for it as they only have 10 morning and 10pm places in each year. Miss d gives good advice on schools but seems to do less string pulling than I thought she might. One big advantage is it has fantastic outside space which almost no other nursery does around here.
Other options: busy bees on Milson road (I heard there were probs there but it's gossip). Really liked it when we visited, bigger and more chaotic than miss d's but nice caring atmosphere. UNless things have changed you wld also need to register soon after birth.
There is one world on brook green (in Chinese church), more Montessori than the others. Also seemed very nice but wants them all day at age 4 (ie second year of nursery) which I didn't want. Uses time to do lots of music etc. can register a bit later.
There is another v small nursery somewhere around Addison gdns which a friend loves but I can't remember the info.
Others go over to Jordan's, just this side of ravenscourt park. Great facilities including its own coffee shop for parents to hang out in! They have all children at 1330 though in first year which again didn't work for us as my children were still napping at 3. Register in first year.
One thought: if you know now you are having 2 boys then presume they are due in the spring? So they will be young for their year and start nursery at 2.5 and go to school at 4.5 - that may factor into your choices.
Clearly we live within 200 metres or so of each other - let me know by pm if I can help more.