Maria,
Ok. I've re-read your 3rd post. I think I have a better picture of what you are trying to do.
Since this involves infants, I would first check for your local and/or federal laws concerning electronics and infant safety.
That said, the power supply you choose would have to meet the requirements laid out by the appropriate authority from a safety stand point.
Having done that, and NOT knowing your total power (Amps times Volts) requirements, I'd suggest using a combination of "D" cells, 2 in series (to provide 3 VDC) for
starters, and then monitor how long they last before the solenoid no longer operates satisfactorily.
There's a lot more to this question you ask then you may realize. But, given that there's the assumption that the solenoid (valve) will only operate momentarily (to relieve the baby bottle pressure differential) and only every so often, I would think this battery arrangement should last a good while.
Can't know for sure, however, with no more information than is at hand for now.
With a tip of the hat to alec_t, here's a simulated circuit you might use that has the solenoid, etc., parameters you've given PLUS an isolated ground recommendation that, I hope, would meet any electrical safety issues.
<EDIT>: (I'll be honest: don't exactly know how to accomplish this isolation. Going to look into it.)
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