I don't think small children have any idea how many grandparents they're 'supposed' to have. You're not the odd one out unless you choose to define yourself that way. There are so many families in circumstances like yours that 'extra' grandparents are the norm. My own grandkids have three grandmas; the kids all know of us as Grandma Firstname. Just tell the truth. They're lucky to have all their grandparents.