No parent wants to force their child to do something about or in which they are unhappy. However, allowing your child to take up and give up pursuits on a whim has the danger of sending out mixed messages.
Ideally, your child should be allowed to participate in the decision to do an extra-mural activity, within certain parameters. Swimming, for example, is a life skill and all children should learn as young as possible. (Although, I am not completely convinced about drown-proofing, that is another debate …!)
Then, if the lustre wears off during the lessons, you should explain to your child that they need to see the school term or month out – in order to give the required notice to the extra-mural teacher (Explain what that means and why: They are expecting that money and need to get it for their income, etc.) and to demonstrate that impulsive behaviour is not appropriate.
Lots of life lessons here – all about courtesy and responsibility, perseverance and self-control, accepting the parent authority, etc.
So, when in doubt say Yes, but …!