Birthday party at a wedding venue - what do I wear?

Im attending a 1st birthday party at a wedding venue in in July, it’s a birthday party so there’s no set dress code, but the venue certainly is more upscale and I can’t decide to what to wear. I’m looking for something summery and flowy like this dress (but not this one) preferably a maxi or midi dress since I’ll be chasing my one year old around. I can’t quite find the balance between dressy and casual yet.

Here’s the venue for anyone interested: https://www.seasonscatering.com/