What about Novation launchkey mk3 25? Connects to iPad or computer to create and around £90
There are quite a few options if you have a computer or iPad to connect to, Native Instruments A25 is very powerful at £150 ish. The thing is, they’re really just controllers, and not synths- it’s the iPad/computer that is doing the work of generating the sounds.
A synthesiser generally has everything integrated and will have some kind of audio output, and although hooking to a computer can open up further possibilities, it’s not essential.
The novation and native instruments boxes don’t have any audio output, and are powered by usb- ie connecting them to another device. They’re controllers really, not synths. That’s not to say it’s a bad idea though, it’s just a more complicated setup that requires other things already.
Hence my recommendation of the Volca, it’s an actual synth and be used standalone or just connected to a simple midi keyboard via a midi lead, and you don’t need a computer/iPad.
OP does your dd have a reasonable computer, or iPad?