mother (honorific); wet nurse (honorific)
Dated honorific term for one's own or someone else's mother, or for a wet nurse. In modern Japanese, お母さん or 母上 are more common for 'mother', and 乳母 is used for 'wet nurse'.
In old documents, 'みおも' was used as a respectful term for one's mother.
In this tale, the wet nurse (みおも) takes care of the young princess.
The reading みおも is likely composed of the honorific prefix 御 (み) and 母 (おも), an old word for 'mother'. The exact derivation is uncertain, but it is an archaic honorific term.