I need a recommendation!

I have a 3 year old, a one year old, and a 7 year old. My 3 year old has significant delays in understanding and communication, and is a runner. I don't drive and need help figuring out a safe way to do the walks to and from school with all my girls. I have a wagon that has seatbelts, but the seats are too close together and my 3 year old can get out of the seatbelt and hurts the baby if they're not separated well enough. I can't hold her hand and walk with her while pushing/pulling the baby in the wagon or a stroller because she's easily distracted and will slip my grip and run to whatever looks interesting. I'm stressed with trying to find ways to keep everyone safe and may be missing something obvious- any ideas help!