The marathon starts at 6AM and it starts outside the park, so you can see them line up for the start. Then head over to the ramp for the monorail from Epcot and you can see them run by that into Epcot (depends on whether they are red or blue route). Then hop the monorail over to the TTC and you can see them run through the TTC. Then hop over to MK and you can watch them run down Main Street and out the front of the castle. You can't go beyond Main Street by the front of the castle. We parked our car at Epcot so we took the monorail back to Epcot and then drove over to AK to see them come out of AK. Then head back to Epcot to watch them finish the race. The finish is outside the park.
The only place you are actually inside the park is at the MK on Main Street and you can enter there without a ticket, but you have to leave before the park officially opens.
Also, if you get to the MK too early on the Monorail, it won't even stop there and will just go on to the Contemporary Resort.
If your boys will be up, ready and at the bus stop by 4:30AM, you can usually ride a bus over to Epcot. But your runner will need to be on the bus by 3AM to get to Epcot. IF you are at a resort with multiple bus stops, the bus usually only picks up at the main bus stop at the entrance to the resort. These bus drivers are not the standard WDW bus drivers and are charter bus drivers who have very little knowledge of WDW.
With your two little ones, you may just want to be there at the end of the race. Depending on how fast your husband is, plan on being at the finish line for his finish, anywhere from 9AM to 1PM. You probably won't be able to get a seat to watch the finish (they have bleachers set up, but seating is very limited and first come first served).
Has he already signed up for the marathon yet? There may not be much time left to sign up. I know the half and Goofy Challenge are already closed.
The MK is about the 10 mile mark for the marathon. If he leaves at 6AM, how long will it take him to get to the MK? That's how much time you have to get there from the start.