I'm sure it will take some time for you all to adjust to your new situation.
My kids are 6 and 2-years-old and they have always played pretty much all day, but we talk and interact a lot and they are expected to help with chores and clean up their messes. Kids can learn more than you think just by spending time with their parents. My just-turned-2-year-old is learning to count, say the ABCs, sing songs, and recognize colors and shapes. My 6-year-old goes to 1/2 day Kindergarten but was learning to read and do math well before he started school. Now he's learning multiplication at home and reading harder words than what he's working on at school. He asks questions and we answer them, and explain how things work and why things are the way they are.
Set up a schedule/routine and stick to it. My toddler gets up between 7 or 8, has a small snack, and watches (I use that term loosely since he mostly just plays) PBS kid shows and plays until his brother wakes up around 8:30, then they eat breakfast and get dressed. They either play and do activities or we go somewhere (usually the gym or running errands) until lunch. After lunch, they clean up and my oldest goes to school and my toddler takes a nap. I'm pregnant, so sometimes I use that time to take a nap myself! That is also a great time to catch up on housework. After naps, it's more play/activity/clean-up time until dinner time. After dinner, they get in their pajamas and we have family time until snack, toothbrush, and bed time. Even though the specific activities may change from day-to-day, the routine is basically the same and they know what is expected of them.
They really enjoy coloring, playing with Play-Doh, building things with blocks or Legos, playing with their train set or car tracks, dancing to music, reading books, etc. They also watch PBS kid shows and other educational videos like Magic School Bus or Leap Frog when Mommy needs some down time.
I have a gym membership, so the boys interact with kids there, and we also do play dates with their friends from church or school (my oldest in in 1/2 day Kindergarten).
We also go on outings to the store when we need to get out of the house, even if I don't need to buy anything. The boys love playing in the toy aisles.
The library is a good, free field trip. A lot of them have story or activity times for free and they are never too young to learn to love books.
Most malls have indoor play areas for little ones.
Trust me, your children with benefit from this time with you. It will not be detrimental to them. Just take the time to get into a routine and teach them valuable lessons that are best learned from Mom. As exhausting/frustrating/lonely as being a SAHM can be, it can also be a wonderful, rewarding experience for the whole family.