There have actually been quite a few presidential children married while their dad held office. I’ll refrain from discussing Jenna Bush’s wedding in detail because it falls within the last 20 years. So we’ll go back a bit further.

• Dorothy Walker Bush. She got married at Camp David in 1992. It was a small private affair, only 130 invitations were issued. And they did not include political allies. VP Dan Quail, for example was not invited. Nor did the press attend.
• Patti Davis (Patricia Ann Reagan). She got married at a Hotel in Bel-Air in 1984. Again this was a small private wedding, again only a bit more than 100 invitations were issued. There was a small press release with a few photos. Otherwise it wasn’t shared with the public.
• Maureen Reagan. She married in a larger ceremony in a Hotel in Beverly Hills in 1981, with 375 guests. Neither Ronald or Nancy appeared. He was recuperating from the near fatal assassination attempt, and there was no love lost between him and his first wife Jane Wyman. In fact neither of her half-siblings showed up either. The only sign that this was a “presidential” wedding was the very visible Secret Service contingent.

It should be pointed out that all three of these were second or third marriages. None of them were shared with the public in any particular way. Nor did they gain much interest. The next one back was a completely different thing.

• Tricia Nixon. She got married in the White House Rose Garden in 1971. She was given far more the princess type treatment. Even before her wedding she had appeared on the cover of Time, Life, Rolling Stone, even teeny magazines like American Girl. She was marrying her college sweetheart. It was a great romance. They invited 400 people and even broadcast the wedding live, much like the Charles and Diana royal wedding. It wasn’t grand on the scale of a British Royal wedding. But it was a lavish affair to which the public was invited to attend via their TVs.
• Lynda Bird Johnson’s 1967 wedding in the East Room was a smaller more private affair.
• Luci Baines Johnson’s wedding in 1966 however compares to Tricia’s. It was held in the Shrine of the Immaculate Conception. It was also publicly broadcast, and with the grandeur of the church may have outshown the garden wedding.

Prior to that we go back to a pre-tv era. Earlier weddings were generally held in accordance to the social status of the families of the bride and grooms. Franklin Delano Roosevelt Jr. for example married at his In-laws’ church and then had a reception at Owl's Nest, their estate. Mostly though, White House children tended to take advantage of their position and frequently married there.

So the short answer is, a couple have been big national spectacles. Mostly though they have been private family affairs.