Night in Atami before flying out from Haneda?

Hi, We have our outbound flight from Haneda on a Wednesday at 12:40 in the afternoon. Is it feasible to spend our last night in Atami (or Ito) and catch a train to Haneda in the morning or is that too stressful?
When should we plan to arrive at Haneda airport?
We also have a baby with us so prefer not to be too rushed! The alternative i guess would be to spend the last night in Tokyo but we�ed love to take it easy at the end and rather finish in a nice seaside resort.

Very much doable. It is only 80 minutes from Atami Station to the Haneda Airport by shinkansen and Keikyu via Shinagawa.

For international flights, I recommend to arrive at the airport around 3 hours before departure. That means the 8:44 shinkansen departure from Atami. You could even have breakfast at the hotel, if your hotel is not too far from the station.

I would not do that if I were you, especially because you're with a baby and say that you "prefer not to be too rushed."

Moving around with a baby means that you always have the risk of taking unexpected breaks and even starting all over. Your baby could get restless or sick, and you tend to carry a lot of physical things for this little person that you may accidentally leave behind and go back for. Or if it so happens that the trains stop due to some accident or sudden change in weather, you will not be able to get to Haneda unless you pay a huge amount for a taxi.

Also, Atami and Ito, in my perspective, are not "nice seaside resorts" in the sense that it allows you to relax on your final night, especially if you're leaving so early. Instead, why not stay at bay resorts such as Yokohama or even the bay area of Tokyo. Both areas offer direct Limousine Buses to Haneda Airport. On the day before you leave, you can even do a day trip to the Shona area (like one of the beaches along the Enoden train line) if you want to relax at a beach. The area is very scenic and sophisticated.

Additionally, Haneda Airport itself is very updated and offers lots of shopping, dining and a bit of amusement, so you may even want to consider arriving earlier at the airport or even staying overnight at one of the hotels nearby so you can watch the airplanes.

I hope it helps.