With kids, it depends on the day.
The one angle that reframes Lisbon: from the water you finally read the city as the tiered amphitheatre it is, with Belém Tower, the 25 de Abril bridge and Cristo Rei lined up along the shore and the low gold light turning the whole front to amber. The catch: the big multi-deck tour boats feel padded and pricey for the distance, the commentary is canned, and you're packed in. The version worth paying for is a small sailboat (a licensed operator such as Palma Yachts runs ~2-hour sunset sails from Doca de Belém by Padrão dos Descobrimentos, drink included) — quiet, intimate, wind in the sails. And if you only want the view, not the experience, the public Cacilhas ferry from Cais do Sodré gives you a slice of the same skyline for a couple of euros.