Hi, Jaclyn!
Let me detail this all out for you, as it can get a bit confusing! Let's start with your 4 year old. Kids ages 3-9 will need a park ticket and will officially be on the
Disney Dining Plan with dining credits. They can order any meal (adult or child) at Quick Service locations, but must order from the kids menu at Table Service locations.
Your
2 year old will not need a park ticket (hooray, FREE entry into the Disney theme parks! Enjoy this perk while you can!) With Quick Service meals, this little mouseketeer is welcome to share your meals with you, or you can order a separate meal but there will be a charge for it (or you can use one of your dining credits). Ask for an extra cup to share a drink, or cups of ice water are always free.
At Table Service locations, your child is welcome to share off your plate. If you order an extra meal, you can again use one of your dining credits or pay out of pocket. An easy option is to choose buffets or family style dining where they can eat all they wish for no additional charge.
Snack credits have the same rule...since your 2 year old has no credits of his/her own, he/she can share your snack or use your credits for snacks.
Lastly, your 2 year old will not be eligible for the Refillable Beverage Mug that each guest on the Dining Plan receives. This should not be a huge deal, as you can easily share the beverage when at your resort.
So to sum up, while your child is basically "free" there are no credits received to your account for meals or snacks. You can share your credits, share your snacks and meals, or pay out of pocket for extras. Or, you may do a combination of these options. See what works best for your family!
Cathy