In English, a синица is a type of bird called a "titmouse," "tit," or "tomtit." Use of a direct translation would sound awkward at best, obscene at worst, since "tit" in English is rude slange for "teat" or сиська, сосок. Especially if you try to translate the gender-specific "сестричка" as tit-girlfriend; that would be bad :(
You could use the name of another specie, like "lark," or "sparrow," which are small birds commonly found in childrens' tales. Or you could just apply the generic "birdie," which is a term only a child would use to describe a small bird.
If you need a phrase that would make a good toy brand name, here re a few creative variants you might try: "birdie-buddie," "parakeet pal," or "finchie-friends." I like the first one best.

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Note added at 1 hr (2008-07-19 18:44:09 GMT)
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Angela's note above (I didn't see it there earlier) is spot on. Chickadee-trickadee is pure genius. I would even suggest just "trickadee" on its own, as a new word that perfectly conveys that these are little birds that like to play tricks. Nice job, Angela.