My grandosn could ride Soarin at 5. If the younger child is 4 or older, consider putting them in one of the childcare centers one evening (one at an EPCOT area hotel like Sandcastle club) and then take the others with you to Soarin and Test Track. You could even plan to do this for your "normal" EPCOT day, pick up fastpasses in the afternoon for later that evening, drop the younger ones off at a kids club and thenm hit your fastpass rides.
The cost for the club is $11 per hour, 2 hour minimum. The younger child will feel like they got to do something special--the clubs are fun--my grandson thinks they're another "attraction" designed just for kids.
You could actually do this for the MK also-. Place the younger one a club for two hours and take the others to MK via monorail (if at a monorail hotel club)