Here's another one: http://www.amazon.com/Sauce-Container-Tomato-Bear-Bottle/dp/B005KGEKH8/ref=pd_sim_hg_15

I would put the mayo/mustard in a little container like these http://www.amazon.com/Sauce-Container-Piggy-Square-Bottle/dp/B002TZ04FK/ref=sr_1_sc_2?s=home-garden&ie=UTF8&qid=1383616490&. Pack that in the lunch and have your kid put the mayo on the bread before he eats. Some kids like to feel like they are "making their food". We do that with salad dressing or ketchup and they love to put it on by themselves. If they use a lot, I would save the little plastic containers you get from take out and use those.