‘Let her soak first, you’ll soap her afterwards,’ said Van feverishly.

‘Yes, yes, yes,’ cried Ada.

‘I’m Van,’ said Lucette, standing in the tub with the mulberry soap between her legs and protruding her shiny tummy.

‘You’ll turn into a boy if you do that,’ said Ada sternly, ‘and that won’t be very amusing.’

Warily, the little girl started to sink her buttocks in the water.

‘Too hot,’ she said, ‘much too horribly hot!’

‘It’ll cool,’ said Ada, ‘plop down and relax. Here’s your doll.’

...........................................

Brian Boyd (  The Nabokovian, 53, Fall 2004 ) in his afternote on page 74 wrote :
'Lucette´s next brush with Van and Ada making love, the next day, involver her in a bath with her 'fetus-sized rubber doll", in pointed echo of Aqua´s "fetus...of rubber...produced in her bath."  After Aqua´s delivery, Marina´s son is brought as a substitute and registered as her son Ivan Veen.  And in I.23, standing up in her bath, Lucette lodges a cake of mulberry soap in her vagina, and declares " I´m Van," as if another substitute Van '. 

I would like to comment about the suggestion that "Lucette lodges a cake of mulberry soap in her vagina", as if the child were attempting to mimic a penetration. In my opinion VN´s sentence: " with the mulberry soap between her legs and protruding her shiny tummy..." emphasizes Lucette´s childishness and innocence while it describes her attempt to imagine herself with that wonderful appendix, a pinkish and purple male member like Van´s. 
I don´t think she would have been able at that tender age to register anything further  than the fascinating discovery about a boy´s (Van´s) penis.  

This observation doesn´t mean to contradict in any way Brian Boyd´s very striking parallel bt. Lucette playing with a rubber doll in her bath and Aqua´s delivery of a dead baby who was then exchanged by little Van. My intention is solely to emphasize Lucette´s protuding tummy and her simple perplexity after seeing a male erection.

We know that VN never added details at random and I think that his choice for "mulberry soap" must have been motivated not only by the color but because of a very common children´s song that is used to teach little kids to wash themselves (  and this particular song came to my attention when I realized that Elizabeth Taylor in the movie "Who is Afraid of Virginia Woolf" was not singing to Disney´s " Who is afraid of the big bad Woolf" tone but humming "here we go round the mulberry bush")