Good meeting places in London

Hi there, hoping someone can help me with this. We are arriving in London from NZ in July and are wanting a place where we can meet friends etc. There will be about 20 people coming over the course of about 2 hours (coming and going), and we have a toddler and maybe a couple of other kids coming so it needs to be somewhere wih space for them to crawl around etc. We are staying in Bloomsbury but anywhere in central London is fine. I was thinking of maybe a picnic in Hyde Park and also a wet weather venue...... does anyone know of an easily recognisable landmark in Hyde Park for us to meet? and a large cafe appropriate for kids nearby if the weather is wet? any other suggestions for places would be gladly received - maybe one of the department store cafes? really the most important thing is space for our toddler to move around without annoying people as we may be there for a while. thanks heaps in advance oh and it will be a saturday if that makes a difference - most likely lunch time ish

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