Advance apologies to Croydon, but yes I would avoid this area if I had the choice (& you really do have lots to choose from) for a whole week.
I would consider the prospect of train strikes, if you are reliant upon public transport, perhaps might be wiser, cheaper and easier to factor in transport other than rail? Elizabeth line most definitely is your friend here...
Stratford: Massive shopping mall and Olympic legacy park -not quite sure what you'd reasonably and safely do after day one there tbh, I find it very soulless there and unpleasant.
Greenwich: You have vast green spaces aplenty, lots of indie type areas with access to Greenwich peninsular, galleries, the Thames (could do a riverboat ride and catch some sights and do an afternoon at the south bank?), great museums, the Observatory and affordable cafes, restaurants etc. The winter light show at Canary Wharf (accessible via DLR) is a short journey away...
Also adjacent to Blackheath and there are a few hotels that are not too crazily priced and family friendly right in the centre of Greenwich, meaning all would be walking distance for you.... There's also a Premier Inn in Lewisham which less pretty is a single short bus ride into central Greenwich so could save yourself some cash there if it's just a base you need?
Crystal Palace is pretty but small in options. Great park yes, but very limited high street. Other than eat there's little else to do...
Croydon: As above I am sorry but noooo
I would also give some of the major London hotels a bit if a search as hotel occupancy is very low here right now, you may grab a great deal on a family room somewhere more central?