It stinks when timing/acuity doesn't work out during your orientation. Unfortunately, all you can do is go out of your way to learn whatever you can, and keep moving forward.
Not sure if you're at a facility that does peds surgeries, but once those resume, you'll probably see an increase in volume and acuity. At my hospital, PICU was down to < 1/4 census for over a month, but now there's a huge backlog of surgeries and procedures so they're slammed.
In a sense, it may be better that the major covid shutdowns occurred at the beginning of your orientation rather than at the end. For now, you really should be focusing on the basics, and soaking up whatever knowledge you can.
Even if things don't pick up, try to be kind to yourself. Your situation does stink, but I'm sure that your coworkers will be understanding. People recognize the current circumstances; if there are certain therapies you don't get trained to (ECMO, CVVHD, etc.), they'll make time to teach you whenever they come up. If the unit has a super-sick kid once you're off orientation, maybe you could ask to have a "buddy" shift to take that patient along with a more experienced nurse. I'm sure you're not the only one in this boat.
For now, try to learn as much as absolutely possible given the current conditions. If anything exciting does happen on the unit (like a code or a procedure), be proactive about watching/participating. Ask your preceptor if they can teach you about the pathophys of the most common conditions/diseases on the unit, even if they're not currently present. Depending on how your unit does code carts (I.e. if they restock/lock their own), perhaps they could let you go on a code cart scavenger hunt.
Also, If you have any downtime, read through the policies for the therapies that you haven't been exposed to yet (I.e. ECMO). Once you read through them, you can ask your preceptor questions, and maybe they can show you where to find supplies, how to set things up, etc. When I worked peds cardiac ICU, the nurses would occasionally have to cover for the unit secretary if we were short. Over a couple of 'secretary shifts,' I read through our entire policy book (30+ policies, 200+ pages). I ended up learning things that some of the most experienced people on the unit didn't even know, and it helped me figure out which questions to ask.