I think it's just the usual は vs が difference. In addition to being the topic marker, は is sometimes used in a "contrastive" sense, i.e., if you want to make a statement about something and show contrast with other things, you would use は. ことはできる could imply something like "(don't know about other things), but (someone) can do this thing". This is why a topic presented with は can often be literally translated as "as for...". When it shows contrast, it would be something like - 私はできる= "(I don't know about other people, but) As for me, (I) can do it". On the other hand が is the subject marker and can put emphasis on the whatever it is marking. ことができる can imply "(something) is the thing that (someone) can do", emphasis on こと. When marking a subject like 私ができる it could be literally translated as "(not someone else, but) I am the one that can do (something)".
Another way I've seen it compared is that in sentences with は, the focus is on what comes after it, while in sentences with が, the focus is on what comes before it. Underline=focus ことはできる ことができる
In your example, you'd want to write どの子供でもそんなことができる if you want to put some emphasis on そんなこと. In the other version, は is just a normal topic marker and it can imply contrast.