How very strange! Where would the clothes be - were there other children there who you think they might have got mixed up with?
Or do you think your ex/MIL are making a statement on the way you dress your dd?
Since she had such a lovely time I guess it's not worth that much fuss over; next time get her a special holiday wardrobe from the cheapest of the cheap shops, you could probably do it for 20 quid