Alice, I have not made the chocolate chiffon or white chocolate buttercream recipes that you mention, perhaps someone who has will chime in?
From the Cake Bible, I do like the chocolate fudge cake recipe very much, and have served it with vanilla mousseline buttercream (Cake Bible’s mousseline bc with a tablespoon of vanilla extract per 4.5 cup batch), it is always very well-liked. For the chocolate filling, I love ganache, or if that is too strongly chocolate, then a chocolate buttercream.
For the cylinders, I would really recommend a roulade (roll). If it needs to be a yellow cake, then biscuit roulade from the Cake Bible (or one of its flavor variations) would work well. If this is for kids, I think they will find the swirl pattern of the roulade fun and appealing. If they don’t like buttercream or chocolate, many kids love fruit- how about filling it with a barely sweetened fruit puree? I think the Cake Bible’s strawberry puree lightly sweetened (as for the sauce) would be ideal, just add a little melted gelatin or agar (the agar will have more holding power at room temp) to thicken it. If the kids like lemon, lemon curd or lemon illusion would be wonderful.
If the cake will be kept refrigerated, you could also consider a barely sweetened whipped cream, stabilized with cornstarch or gelatin, for the filling. For instance, yellow roulade with gelatin-stabilized whipped cream and maybe chunks poached pears or peaches. Or add lemon zest to the whipped cream and use fresh blueberries.