When she could sit up well on her own...I think she was about 8 months when I started feeling comfortable with her in the big tub. Now she swims in it. She loooovvvess bathtime.
We always used the big the tub but #1 was using it alone by 5 months and #2 was just in there with #1 from the beginning. For #2 we put him on a hand towel laying down and then once he could sit I think he sat on the towel for a bit but not very long.
Re: Big Bathtub
We're still using the infant tub (he's 14 mons). He still sits in it, I feel its safer/less slippery, and it uses less water. Still working for us.
P.S. Your siggy pic is PRECIOUS!!! She's adorable!
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Harmony Doula
Ditto this (before that I was tag-teaming in the infant tub).
Same here, made the transition to her being in there alone (with our supervision) easier.