I'm thinking of trading my cheap Walmart bike for my aunt's bike.
Would your aunt prefer your bike?
Yes, she really wants the cheap Walmart bike no matter what anybody tells her.
It is fine for you to trade bikes with your aunt. It is possible that your aunt is less concerned with the price of the bikes and just wants the one she feels more at ease riding.
I think it's fine.
It sounds like your aunt understands the trade and just really wants your bike so this seems like a fair trade.