memories of phrases and expressions the old folks used around us are just wonderful, aren't they?! Sorry I can't help you with the Polish question but I want to share some humor from my husband's Polish mother, who used an expression around he and his brother. My husband, for all his childhood, thought his mother was uttering to herself "praising the Lord for such wonderful sons" when in fact, he learned later on the translation for what she was actually saying was "God give me strength"!
Regarding the expressions your great grandmother used around bath time seems to me, similar expressions I recall from my own parents. They weren't really words, they were simply expressions to mean "splash - splash". My parents used many "made up" or imitated expressions; for example, bouncing a child on the knee and saying "lup a ty, lup a ty" like a galloping horse. I hope this is of some help to you. Perhaps someone else will have a different perspective.
Enjoy those wonderful memories!