Millie, 31, shared a picture of the trio on her Instagram page in which she could be seen wearing a ruffled dress pink and blue dress while Hugo had on a pink shirt.
Sienna had a pink bow in her hair while she wore a dress in the same colour.
Hugo, 34, and Millie even got in pink champagne for the occasion.
She captioned the social media snap: ‘Decorations fit for a princess! Thank you @elarievents for creating this dream backdrop for Sienna’s first birthday.’
Millie’s celebrations come after she recently opened up about giving birth to her fist child, and struggling to love her postpartum body.
The television star welcomed Sienna last May, and told Woman’s Health: ‘It was honestly quite hard to see my body after a baby. And I think it is for a lot of women.
‘It can really feel like a shock and something that’s hard to love, maybe – and hard to feel good about… I did feel self-conscious when I went out and wouldn’t want anything to be tight to my tummy.
‘And, yeah, I did feel a bit ashamed of my postpartum body. And it’s kind of sad that I felt that… It feels good to admit that and say, yeah, I did struggle looking in the mirror for the first three months.’
However, Millie also revealed she now loves her new shape, adding: ‘I really do.
‘It definitely won’t go back to being exactly the same as before and I’m okay with that… I feel more womanly, and I’m really loving it.’
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