I've flown fairly often on easyJet with my young kids - I've found they're one of the more kid-friendly airlines. In my experience, they always board families with young children first (although maybe speedy boarders go ahead), so you will certainly be able to sit with your kids. I've never had any problem. And easyJet cabin crew are generally very active and energetic in getting everyone seated and settled quickly - if there is any problem, they'll sort it out.