How would you do it? 6V SLA charging circuit to be built into a ride-on toy

[Brumby]

I don't have a dumpster room like Dave, but when driving around you may sometimes find offerings left by the kerb - which is essentially saying "free if you want it".  (While common many years ago, this is technically "dumping" unless it is an organised waste collection - but it still happens.)

As it happens, I came across a toy ride-on backhoe - the same as the one shown - that seemed in pretty good nick, but there was no charger to be found. So I took it back home and charged the 6V (4AH, I think) SLA battery to see if it worked - and everything did!  It has sound effects and plays music as well - not great quality, but quite loud.

Current charging arrangement is - connect a plug pack which goes straight to the battery.  I'd like something a little better controlled, so unattended charging does not result in premature battery degradation.

I've had a look around at various options from pre-built modules from China to a DIY lash-up and thought it might be worth asking here to see if you have a favourite approach.  The solution will need to be pre-set as it will be placed inside the toy, with no access.  Power supply will be one of the 12V plug packs I scored from Jaycar.

So ... I am interested to learn: How would you do it?

[NiHaoMike]

Adjustable buck converter module with current limiting and a diode in series with the output.