Hi desperate - i think the key question is do you think he would want full custody? i separated from H at the beginning of the year and our situation mirrors yours, even the age of the children. I was panicking he was going to take the children and i would have lost my marriage & children all in one fail swoop.
I suppose the difference is that he wanted out, not me...
Anyway, i took legal advice after he'd been gone a few weeks (i was still off work as was struggling massively) and the solicitor stated that as i was still in the family home (H at his mums) and i was providing for them and represented stability, i may not have been the primary carer as such but there was no case for him to take them. If he suddenly started working, got his own place etc that may have meant further discussions.
However, fast forward 7 months and he's still not working, still at his mums (although OW on the scene a little bit too quickly!!) and we've settled into a routine of him having the children a couple of days, one overnight and every other weekend.
A situation recently highlighted just how much this set-up suits him and how, when it's not 'his time', he couldn't give a shit. He is at heart LAZY and to this day uses the children as an excuse for why he can't work. I think the effort to do more is just beyond him. Especially as, like yours, he never did the night wakings, the cooking etc - massive dose of reality there!
sorry, bit of a rambling all over the place post but please feel free to PM if you like. I know there are differences but the thought of him taking my children was almost more than i could bear so completely know how you feel 