First, you are a star for taking these poor wee ones in. Still I would just note that you do want your friend to keep an eye out for the mum cat since they will often leave the kittens to hunt and it is possible that she may come back for her kittens (unless she has been gone for days and then we'd have to be concerned what became of her).
Now I have to warn you that the milk you are giving is not really appropriate for this situation. TopLife as the company notes is a "daily treat ... made from lactose reduced cows' milk. (More Info ). It is not a replacement for mum's milk. It can be used as something to mix into their food to make it more appealing in later stages of weaning. But it isn't going to meet their nutritional requirements if you are feeding it at a dose similar to what we'd feed to kittens that were on the bottle or early weaning (mostly on milk, just getting into wet food). And it is important to note that using it as such is very likely why they have diarrhea. So, if these kittens are mostly on milk (and not mostly on wet food) at this stage, then you should discontinue this and instead use a proper kitten milk replacer formula. Good quality kitten milk replacer brands include KMR and Cimicat. These can be purchased OTC at vets and pet stores.
Now while the milk is very likely to be the culprit here, we do have to appreciate that there are a plethora of pathologic reasons for diarrhea in the kitten. Therefore, if they don't settle with a proper milk replacer then you will need to consider agents like bacteria, viruses, parasites, and protozoa. In that case, if you have not already, you can consider worming against those parasitic causes. To do so in kittens so young, you will want to use a safe wormer like Panacur. Again this is available OTC at vets and some pet stores but you will need an idea of their weight when purchasing so you get the right dose for them.
Further to that, just since dehydration can cause these little ones to deterorate quickly, we do need to monitor their drinking and hydration. To monitor and make sure they are not becoming dehydrated, you can check some parameters at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether they have a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. (They use a big dog but it makes it easier to see and the principle is just the same). If you check these and are already seeing signs of dehydration, then that would be a red flag that you'd want to have them seen by your vet before this becomes an additional issue for them (since it is often dehydration that causes lethargy, weakness, and makes them feel poorly).
Finally, just to note some safe options to slow their diarrhea while you swap them onto a proper milk source, we do have a few options. The ones we most commonly use are Kaolin/Kaopectate (More Info/Dose) available from your local pharmacy or Protexin Pro-Fiber (which is available OTC at vet practices; example). Do avoid using PeptoBismol or any anti-diarrheas that contain aspirin or Loperamide (as they can cause adverse issues).. In regards ***** ***** safe options, they both will slow diarrhea and the Pro-Fiber has the bonus of providing support to the delicate good bacteria of the GI. So, you can consider trying these as a short term means of trying to soothe their GI upset and slow that dehydrating diarrhea.
Overall, diarrhea in the kitten can be caused by a range of agents. That said, the first point of call is changing them onto a more appropriate milk source since cow milk (even low in lactose) is not ideal for barely weaned kittens and is likely the cause of their diarrhea. Further to this, you can consider worming now and the above supportive care. If you do so, but they are not settling over the next 12-24 hours or are showing dehydration, then we'd want to have them checked by their vet. The vet can help pinpoint which of the remaining causes could be present and dispense antibiotics, treat with fluids to resolve dehydration +/- anti-diarrheals to get them settled (which should stop the blood as well) and back on track for you.
I hope this information is helpful.
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All the best,
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