Close family and friends were invited to celebrate the occasion at the Windsor residence, where a cake by the Duke and Duchess of Sussex's wedding baker Claire Ptak was also served.
A spokesperson for the couple did not name specific family members who attended the event.
Harry and Meghan attended Friday's thanksgiving service and were seen observing Trooping the Colour, but largely kept a low profile over the bank holiday weekend.
The picture of Lilibet, which shows her smiling and dressed in a pale blue frock with a white bow in her hair, was taken by photographer and friend of the couple Misan Harriman, who attended the picnic and was seen in one of the images.
Another image showed Meghan and Lilibet with other party guests.
Harry and Meghan thanked well-wishers, including some who made donations amounting to over £79,000 in Lilibet’s honour to the World Central Kitchen, a not-for-profit organisation partnered with the couple’s Archewell foundation, according to a spokesperson for the Sussexes.
The organisation has provided support in Uvalde, Texas and Ukraine, the spokesperson said.
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