Well, it is crowded, which is the best recommendation to stay in town and not just come in for the day - the train, parking, finding a place to rest for a while is nearly impossible. Get yourself a base to work from, get to know your way around during the day, and then scout out places to watch the parade from. The only times it gets really packed are the nights of the big parades (Sun and Tues). Monday is a quiet day and might be a good one to take the train into Barcelona for a ride on the tourist bus.

The best setup I saw was one a few FTers had in 2004. They had a room in the Hotel Subur that looked out over Passeig de la Ribera and watched the whole thing from their terrace. They had a great birds-eye view, stayed literally above the crowd, and were definitely warmer than the people on the street. The hotel isn't luxurious (2-stars) but it's clean and the service is friendly. They still have availability for that weekend starting at around 50EUR/night.

Do note that Carnaval is NOISY and the beach in front of that hotel will be packed with people, live music, etc. all night long during the weekend. Don't plan on a restful sleep. If you can swing it, book the Subur to watch the parade and then go back to the Subur Marítim or the Meliá to crash.

Even if you don't get a room, he'll be able to see OK. Last year we were able to get a front-row vantage point just a couple of blocks from the church. There were lots of kids on shoulders. Most of the floats are big and up high, so they are easy to see.