If you choose London as your other city in addition to Amsterdam, do take into account that you will want to fly open jaw into London and then fly out of Amsterdam. If you do the reverse and fly home from London, there are some extra fees you'll have to pay which will jack up your airfare.
London has a lot of major sights that are free admission, so that can help offset some of your costs. With a toddler, do try to stay as central as you can afford. There are London lodging options that are quite affordable - budget business-class hotel chains like Ibis, university lodging (ex. London School of Economics has both dorm rooms and apartments they rent to tourists with locations all over the city), or consider some of the private, ensuite family rooms at hostels which can be really good since you'll have access to a kitchen, laundry facilities and various lounge areas where your family can stretch out instead of being crammed together in a single room. Some of the newer "boutique" hostels are really nice and some chains (like those run by Hosteling International) are very popular with families. Read over the reviews on sites like Hostelworld.com to hear what other guests have to say about the hostel - avoid the ones whose websites or reviews tout a "party" atmosphere.
Since you just recently went to Paris, I'd suggest London for 5-7 nights (maybe include a day trip outside of London in the middle of your stay), then train into Brussels and grab one of the commuter rail trains that goes to a place like Bruges or Ghent and spend 1-2 nights. Belgium has some nice smaller cities and a place like Bruges or Ghent can be a good place to unwind a little as they are compact and very walkable - and prices for lodging and food are pretty reasonable. Catch the train to Amsterdam and spend about 4-5 nights (depending on how long you want to stay in London).
