I went when my first was 9 months then when my second was 6 months and first 2years. It was utterly magical. Some of my most treasured memories come from Disney as they have grown up. They are both teens now, veterans of very many visits and total disney nerds.
YY to sling. You'll probably find a lot you can go on without needing baby swap (at that age we purposefully spent the whole time with the kids), but baby swap works remarkably well (I still do it as DS(14) has SN and cant be left alone but cant ride everything Dd(16) can). I took a bike lock on occasion but never bothered with it but then I took a battered old maclaren strollers (I had one that would lie flat).
I found staying on site was best for us. We do most of our ride riding in extra magic hours and before lunch and then shows and parades after lunch. Our favourite hotel is the Sequoia lodge because it is a beautifully chill hotel, the right balance of walk to the parks (personally I'm not a fan of shuttle buses) and price for us. But I think all the Disney hotels have their own benefits so it depends what you want really. We didnt find the actual Disneyland hotel inherently magical but the location inside security the whole holiday and the room service breakfast definitely is!!! I have heard good things about the santa fe and Cheyenne but they are too far for us to walk (I'm pushing a wheelchair). Newport bay is very bright and offers room service breakfast. People tend to either be Sequioa lodge people or newport bay. Newport bay people tend to appreciate the airiness and find Sequioa a bit old fashioned and fusty. Sequoia lodge people tend to find NPB a bit brash and wonder what the extra cost is about. I'd love to try the new marvel hotel but it's the wrong room layout for us.
Some people return to the hotel after lunch but at that age I'd put them down for a sleep in the buggy while we got a good space at a show or parade. We pick off peak times when the parks close earlier knowing that we value the early mornings and end up too knackered doing both ends of the day!!
Food is awesome. Even the fast food has improved in recent years. If you can get character dining do it but it's been suspended during covid so far. I have some incredible photos of the kids growing up in Cafe Mickey. We do the full board meal plan which some people say is too much food, but is pretty incredible value for money, we like a break at the middle of the day and you get to eat in some of the most beautiful Disney themed restaurants. Also If you eat at buffets it can be quicker than the counter service if you catch them at the wrong moment.
Dont plan to do the evening show every night, your extra magic hours in the morning are too precious!! Once is enough.
Watch some key Disney stuff before you go. Definitely include something Mickey mouse (probably clubhouse), toy story (1&/or2), lilo and stitch (stitch live in the studios is beautiful), Ratatouille, peter pan, any of the starwars (the cartoons are good), cars. It can be fun to set up a weekly disney movie evening.
You need at least 3/4 extra magic hours. If you stay close by first night you can get extra magic hours on your first full day (so 4 extra magic hours for 3 nights stay)