You’re definitely doing the right thing. I’m also doing AGF white year this year but since I have older kids it was recommended that I read the older kid’s spine and just read the picture books to my younger ones. I think that is plenty for my 5 & 8 year old. We all learn so much from picture books! I hope you have fun with American history this year -that’s what matters at your young ones’ ages!
You can also read the Cornerstones of Freedom series, which basically give you a story of events. The Story of the Constitution, The Story of the Alamo, etc.