Except the press aren't professional, W&H know that better than anyone. Previously the press suspected the two men would commemorate the day with the Spencers in some fashion, now with the press release the press knows Catherine and the kids are involved and their interest has skyrocketed. The press will also now be desperate for Meghan to attend as this is now seen as a "official" family gathering thanks to the KP letterhead, her attendance will be used to fuel their imminent engagement articles.
W&H know they get some interest from the media, they also know that Catherine, George, Charlotte and Meghan get way more interest than the two them. This release has now guaranteed that the press will stake out the roads outside Althrop so they can get car pics of the arrivals. Members of the public will also probably hang around the roads hoping to catch a glimpse of the royals now that they know for certain five royals will be there.
Like I said, all this press release did was incite a media circus. Unless W&H are complete morons, they realized this would be the outcome. So this was done intentionally to bring a spotlight, to a feigned private event.