This is very normal. Guinea Pigs, as prey animals, are naturally scared. Two days is such a short space of time but it can actually take weeks, sometimes months for them to settle in. They will settle though.
Do explain to your children though that, as prey animals, they may never be cuddly. Some enjoy it in time, some tolerate it and some never will. The best thing is to leave them for a week, let them settle in and then start to offer food from your hand. Once they take food from you, they are starting to trust you. This is fantastic when they start doing this as it means you have bonded with them, you dont see you as a threat and are happy to be around you. This part of the process can take quite some time and it varies from piggy to piggy. It took one of mine six weeks to settle in and take food from me. From then on he was happy to stay out and about to be around me. However, the other one is much more anxious and it took him 18 months before he took food from me and stopped running away from me. It took a lot of time, patience. They've been with me for three years now and they are both happy to be around me, I can sit in their shed with them and they will going about their piggy business around me and are happy to spend time with me. However, they are not cuddly pigs - they will tolerate a short amount of handling for their weekly health and weight checks, hair and nail clipping and if I need to give medication but they do not like just being held for a cuddle.
The guides below are a collection of new owner guides which you will find very useful. There is a lot there, but do take the time to read through them.
New Owners' Most Helpful How-To Guides and Information
These guides below are specific to settling them in and how to pick them up
Arrival in a home from the perspective of pet shop guinea pigs
Understanding Prey Animal Instincts, Guinea Pig Whispering And Cuddling Tips
How To Pick Up And Weigh Your Guinea Pigs Safely
In terms of cleaning the cage, clean the area where they are not, then have them move to the other side of the cage and then clean that area. Do it slowly, talking to them the whole time - predators dont announce their presence so if you talk as you approach and talk while you are going about your cleaning etc then they will learn that you aren't going to hurt them.