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According to JTB's website, the
Prince Park Tower supposedly has "barrier free" rooms and connecting rooms. Whether it has "barrier free" connecting rooms, I don't know. Might be worth calling.
Actually, just looked at the Prince Park Tower website -
That universal room looks like it might fit parents and a 6 & 7 year old ... Definitely worth asking. Scratch that. The beds are only 1.15m wide. Looked wider in the first photo. Still, worth asking about connecting rooms, I guess.
Failing that, LapLap might have some advice to offer.
BTW - August in Tokyo is stinkin' hot and humid.