“Do you think she is too young to be my flower girl” That all depends on the maturity and temperament of the child and whether you think she will cope the whole event. If you’re going to choose a toddler, it’ll definitely make things easier if her parents are in the wedding party or sitting in the front row, then at least mom can be there to encourage her down the isle or to make a swift exit if your niece decides that the processional is a good time for a tantrum.
At the end of the day, it is up to you to decide. Can you handle a little deviation from the plan should the situation present itself? Yes? Then go for it!

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