It's not a 4yr old. It's not "let's all roll our eyes because people have a problem with a 4yr old, hooooow ridiculous".
It's the grown adult mother, putting her in the dress. Obviously. 
No one will have a problem with the child. They'll have a problem with the mother, just like my aunt had a problem with me, until she realised I knew nothing about the bloody "surprise" buttonholes either.
Buttonholes for everyone at your wedding, pointless comment, that was your choice and included everybody. Not for a select few that indicated they were in the bridal party.
Do not make something look like part of the bridal party unless it's been decided by the bride and groom. It's not your decision. Don't be ill mannered enough to ask and put her on the spot and unable to say no.