It found almost half of mothers with a child over the age of one (46%) use toddler, or "growing up", milk, despite health professionals regularly advising parents that a healthy diet including cows' milk provides a young child's required nutrition.
Which? said parents could save at least £500 a year by switching from ready-to-serve toddler milk, costing around £593 a year, to cows' milk, which costs £62.
Government advice is that toddler milk is unnecessary as children can drink cows' milk from the age of one.
A comparison found full-fat cows' milk contains less sugar - 4.7g per 100ml - than Hipp Organic combiotic growing up milk powder, at 7.9g.
The study also found cows' milk contains higher levels of calcium - 122mg per 100ml - than Apatamil 1yr+ growing up milk powder and Cow & Gate 1-2 yrs growing-up milk powder, which both contain 86mg.
The report said toddler milks contain more iron and vitamin D than cows' milk, but these nutrients could be obtained from a child's diet and a multivitamin that contains vitamin A, C and D.
Unlike infant formula, toddler milks are not covered by specific legislation on ingredients.
According to Which?, the formula market is worth around £359 million a year, with toddler milks the fastest growing sector.
Which? executive director Richard Lloyd said: "At a time when so many household budgets are severely squeezed, parents could be saving hundreds of pounds on toddler milks that the Government says are unnecessary.
"Ministers should make their advice much clearer and introduce guidance on the ingredients of toddler milks, including the level of sugar and calcium."
Which? surveyed 727 mothers with children under three years between November 8 and December 6 last year.