Novation Circuit

Soundwise it's at least going to be far closer than the TD3 and RD6, and it can make two different sounds at the same time instead of one, and you can play back all the sounds the RD6 can make and more.

A used Novation Circuit.

A used novation circuit or circuit tracks is easily in budget and very easy and immediate

I'll also throw in that the original Novation Circuit is also a good fit, I like that it has a built-in speaker.

Seen quite a few threads recently about synths & grooveboxes suitable for kids, and by far the most popular & consistent recommendation is the Novation Circuit - I don't have one myself but it looks pretty simple to use, but also not too limiting - plus you can get a beat going on it too, which I imagine would be a big bonus for youngsters!

I'd recommend the Circuit as a first synth for anyone who's into loop-based dance/pop genres, regardless of age, because you can put a basic track together on your own, which is a great feeling as a beginner.

Something like novation circuit or elektron model samples/cycles could be a good complimentary groovebox to add drums and bass.

I’d go with a novation circuit, will let you do a lot more as far as jamming and songwriting and you can expand to a full synth later.