We took 3yo at end of September last year and had a blast! Definitely not too young!
You'll need to decide a bit what to prioritise. Unlike a standard theme park it’s not just rides. There are:
Rides
Shows and Parades
Character Meet n Greets
If you think your wee one would like to see Disney characters by far the quickest way to meet a few is to book character dining, depending on who you want to see this would be at Plaza Gardens, Auberge de Cendrillon or Royal Banquet. At plaza can book breakfast or dinner (no characters at lunch) and see usually some Winnie the Pooh characters, maybe hook or pinnochio, maybe some Mickey and friends. At auberge every meal is with princesses. As an off site guest you could only book the 9.45 sitting not the 8.15.
You can also queue to meet characters but queues can be long. The benefit can be that you are often allowed buggies in character queues (these are never allowed in ride queues) and a tip to meet Mickey is look next to Haunted Mansion, he is often there and we met him after a 20 minute wait (vs 1 hour in his meet and greet building in fantasy land).
DD was surprisingly really into characters. Also if your son likes marvel you can book avengers and spider man meets at the studios on the app- you need to be in the park to book it though, I think bookings go live at 10 and 2 on the day.
If you’re not interested in characters at all that’s simpler, just focus on rides and parades/ shows. There are 3 big shows at the studios, frozen (which is ok) Mickey and the magician (which is fab) and Pixar Together (I haven’t seen but heard good things). I would suggest trying to see one of the latter two. Definitely catch the main parade in the main park, DD was spellbound.
Finally rides, check the app for queue times, bring a good carrier for the little one and something to entertain the older one in queues. Rides like dumbo and Peter Pan get excessively long queues whereas we found the carousel, tea cups and story book boat queues were usually under 30 mins. If you really want to go on a ride and queue is very long you can pay for premiere access and baby will be free, we did this for autopia and orbitron (one adult only with DD) as it wasn’t too expensive.
Finally, not sure if you’re going with another adult but if so you make use of parent swap, this means you can queue as normal, ride a height restricted ride, then ask at the exit for a ticket and your partner can go on without queuing. You could also each take it in turns to take the 3yo on rides the baby can’t go on, but unless 3yo is quite tall this will probably be limited to a couple of rides as the vast majority have no height restrictions and those that do are mostly over 1m. Autopia and parachute drop being the exceptions I can think of.
Embrace it and have a great trip!