Hi,
We went to Porta Pollensa at the beginning of September so here goes:
I would recommend being near the beach but at the opposite end of the main strip (i.e. away from the Marina), probably what you refer to as the pine Walk. The beach is narrower but it's a bit less resorty somehow.
The water is incredibly shallow, if you sprinkle breadcrumbs into the sea shoals of wish will swarm around you. DD loved this.
The public lido is fine but I'm not sure it will be open in early spring. My dad bases himself near Porta Pollensa from April onwards and although the weather is generally warmer than here, by Mallorquin standards it's freezing!
Make sure you hire a car, there's not a lot to do in Porta pollensa if it's raining but lots of places are easy to get to. Some such as Palma are fine by bus, but there's a reduced service until June and the roundabout routes really add to journey time. The buses get really full so it's not fun with a squirming toddler!
There is a decent soft play/ball pool near the main post office. They can also organise baby-sitting. Again, unsure of winter opening times.
Definitely try to rent an apartment privately, but not in Llenaire/Gotmar as these are a bit too far out with a toddler.
Hope some of that's of interest, if there's anything else, post away! My dad is Mallorquin and spends most of the year out there so if I'm not sure i can ask him!