Walk down the milk aisle and you will find soy, oat, almond, rice and more next to the dairy. They look alike in a glass, but nutritionally they are quite different drinks. Whether a swap is "as good" depends on what you use milk for, and on one word on the label: fortified.
Semi-skimmed cow's milk is a handy package: roughly 3.5 g of protein and 120 mg of calcium per 100 ml, plus iodine, vitamin B12 and riboflavin. In many countries dairy is a main source of calcium and iodine, which is why swapping it out is worth a little thought rather than a shrug.
Soy is the closest match. It has protein comparable to dairy milk, and the Vegan Society notes that unsweetened, fortified soy, oat and almond drinks are acceptable alternatives to cow's milk from age one. Oat is naturally higher in carbohydrate and low in protein (around 0.5 g per 100 ml). Almond is very light, often under 20 kcal per 100 ml, because it is mostly water and contains few nuts. Rice has the least protein of all.
That does not make oat, almond or rice "bad". It means they play a different role: a splash in tea or coffee, or a base for porridge, rather than a protein source. If you use plant milk in volume, plan protein elsewhere (beans, tofu, eggs, dairy yoghurt, nuts).
Unfortified plant drinks contain very little calcium, often below 10 mg per 100 ml in published food data. Fortified ones are usually set near cow's milk, around 120 mg per 100 ml, though some brands differ and food-composition tables show a range. Fortification also commonly adds vitamin D, vitamin B12 and iodine, but not every brand does, so read the label. Shake the carton: added calcium can settle.
Same glass, different drink. Choose unsweetened and fortified, and treat soy as the one that matches dairy on protein.
This guide is general information, not medical advice. Plant drinks are not a complete replacement for breast milk, infant formula or cow's milk in babies and toddlers, so ask a health visitor, doctor or dietitian before using them as a main drink for a child under five. If you have a milk allergy, a medical condition, take medication, are pregnant or breastfeeding, or are managing an eating disorder, talk to a doctor or dietitian before changing what you drink or eat.