gatorlisa gave you good advice above. The 30 days you mentioned is normally when off site guests can start to make FP if they already have tickets, but you can start as soon as you get yours. If you haven't already, set up your
MDE account on the Disney site and download app on your phone. As she said, you can then link tickets on there, and pick your first three FP. Hopefully you can get the three for late morning/early afternoon, and then as soon as you check in for 3rd, you can look at what is available for a fourth(and then again for 5th,6th, etc.).
If possible get there early for Rope Drop, to go to popular rides you do not have FP for. Most popular are Seven Dwarfs Mine Train, Peter Pan, and the mountains(Space, Splash, and Big Thunder). As mentioned, Seven Dwarfs may be gone, so you could go there first, as it will have longest lines all day. Peter Pan, crazily enough, is also usually an hour wait, so good to FP or do early. Mountains will have longer waits, then comes Haunted Mansion, Pirates, and Buzz.
Since you have the age gap with kids, you may wnt to split FP, if 3yr. old will not do some rides like Space Mtn, you could do 3 FP for that, and 2 for Winnie the Pooh for example. You just choose the family members from list on your account that want the FP for each ride you choose.