Inviting 3 year old niece

When we first started wedding planning, we asked my FSIL to be a BM and her 3 yo daughter (FH niece) to be the flower girl. Shortly after we got engaged, my FH and FSIL had a huge falling out. She dropped out of the wedding and needless to say her daughter is no longer a flower girl. We did not replace her and are not inviting any children to the wedding. Fast forward 8 months later, no one had spoken and we had not seen them, but we decided to invite FSIL, her husband, and her daughter to the wedding. She reached out and said her and her daughter would be attending and she wants to open up communication lines, which is good. I just want to make sure it won't be odd to have her 3 year old daughter there as a non wedding party member. I don't know if other parents will get upset that their kids weren't invited, even tho she is mine or my FH's only niece.