hi there,
my lo is 8 wks old and on a 3 hour EASY plan. I find that she can only stay awake for 1 - 1.5hrs max (during which she has a feed, nappy change and some activity time) before she starts yawning. When the yawns start, I will swaddle her, and she will protest and start fussing (grunting and stiffening her body), but it's happened enough times for me to realise that its just her way of getting ready for bed (as wierd as that sounds!)
Keep perservering, you're doing a great job, especially with two children!
cheers