A family social event c1970

This snapshot seems to show a family event somewhere, but I wonder what the event actually was? There are some interesting aspects of the photo which probably give clues, but I’m at a loss to know exactly what the event was. For example, the little girl in the centre of the photo is wearing a swim ring and the little boy near her feet has some yellow wellington boots – are they planning a summer holiday?

On the mantelpiece I can see a single card, possibly a birthday card – is this someone’s birthday and the ring and wellington boots are presents? The man is eating some food which could suggest a party.