Is there hospital on Sal? Yes, brand new with europian standarts.
Would you recomend to travel and stay on Cabo Verde with children? Yes, why not? The temperatures are fine for a kids, maybe the water can be a bit cold, but for this reasons we recomend to visit Igrijinha – small and shallow lagoon on the east side of Santa Maria. The black lava stones heated by sun makes the water warm. There are no such a diseases like in Asia or cenntral Afica, so your kid wont be in any danger.
Shloud i pack something special for my trip to Cabo Verde? Yes, suncream! 50FPF is best solution to be safe from very very strong sun and good sunglases too. Otherwise nothing special.
Are there any diseases or is any vaccination is neccesary? No, no vaccination is needed. Only diarrhea is quite usual (so be careful where you go for a lunch) and you have to take care about drinking enough of water (4-5l while you are on the beach!)
How about criminality on Cabo Verde? Well, if you stick to a rules, like dont go alone, drunk and with your iPhone in your hand to a dark streets outside of the center of Santa Maria, it is basically safe place. Bad things happens, but you have to just take care of your things on the beach and dont show to much cash in the bars and you will be fine.