It would probably be best to return the otos. Getting them to eat from prepared foods is not always easy. I won't explain that, but move on to the guppies.
A 30 liter tank (= 8 gallons) can work for guppies; I understand the aspect of your child and a near-empty tank is not going to hold his/her interest, so if you are regular with your maintenance, and careful not to overfeed, and if there is some décor you might bee OK. And before someone jumps down my throat, sometimes there are situations where we would not advise this or that, but when it is already a done deal, we try to make the best case for success. Floating plants which are easiest to care for would also be a good idea.
No mention was made of water parameters, and guppies like all livebearers require some mineral in the water. The GH is the general or total hardness, and gives us this value. You might be able to track that down on the website of your municipal water authority. The pH is related, and should be basic (above 7) for these fish.