It is definately a vacation, just not a very relaxing vacation. I am assuming though you already knew that. I think you should absolutely go. It was one of the best vacations I had. I just went this past July with my two sons- 3 years and 7 months. We had a blast. Your younger one is going to live in the moment, smiling at lights, colors, characters, etc. My 3 year old talks about this trip constantly. He loved it. At three he was young enough to still believe the characters were real, etc. We intend to go back again when the youngest is 3 and the oldest is 6 but I know my six year old will have a different take. Also note, my 3 year old is tall so he was 41" which got him on adult rides to such as splash mountain and thunder mountain. He had a blast. So overall, yes, go. That said, here are some things I recommend:
1. Double stroller...they rent them at Disney but they are uncomfortable and do not recline so bring your own. I had a large 3 year old so I invested in a used Bobs double stroller which was cheaper then renting a double for the trip. Both my kids fit in there were able to nap in there because it fully reclined, etc. In fact, the disney rentals done recline at all so your 8 month old would not be able to use it. You can also bring two strollers of your own if you have enough hands...justs bring ones the kids are comfortable in and they can nap in.
2. get the unofficial guide to walt disney and read it cover to cover...great tips. It will tell you how to work the lines, fast pass, etc. I never waited more then 10 min and in July! I wound making a spreadsheet that I carried with me that told me (based off of the book) best rides to go on at what time, if it took fast pass, how to work the line etc. It really helped. Leverage fast pass which means check your watch and as soon as you are able to get another fast pass, get one.
3. Confirm it is still the case but Fast Passes done expire although they give you expiration times. You can not go any earlier then the first time but you can go after the expiration. One of those 'unsaid' things. But double check when you are there.
4. go to the park early and then afternoon go for a swim and then go back to the parks in evening. I will say we did this some of the times, other times we just stayed and kids slept in stroller at park.
5. bring rain shields for strollers
6. invest in the stroller fans...i bought them online before i went. It really kept the kids cool during the day...I just hooked them to stroller
7. i highly recommend the meal plan
8. make your reservations as SOON as you book your trip. restaurants book quickly. Otherwise you will face a 2 hour wait to eat...with reservations you walk right in. we never waited. This does mean you have ot plan out alittle what parks you will be at when. If you are eating at epcot one evening for dinner, then you would plan on visiting epcot end of day that day.
9. Have fun but dont stress. you may not see everything. We were there 10 days and did epcot, magic kingdom, animal kingdom and hollywood. thats it...we just kept going back to different parks. magic kingdom will most likely be their favorite.
10. Yes, take turns on adult rides. they have something where you dont rewait...one adult goes on and stays with kids while one rides and then you do a switch and the other adult gets right on. the line people will tell you how to do it, just say you have two kids and need to do a switch.
11. One more thing about strollers. Infant car seats are allowed on a lot of rides. If your child is still in one (mine was at 7 months) then bring it with snap and go. at that age they nap a lot. that way you can just bring them on ride as they nap.
hope this helps and enjoy! take pictures!
bottomline, prep ahead and you will have a blast.