Childs 6v ride on toy question

We were recently given a childs 6v ride on toy it is 15 years old and having carried out a few checks found that when load was out on the battery the battery was giving a reading of 0.2v so. Having replaced the battery like for like (ultramax 4.5ah 6v). The new battery did the trick ( we did test the theory out by for only a few seconds connecting a 9v battery to the cars terminals and the motor powered up. We now have the new battery and following a charge period of approx 3h hours, the battery can be heard bubbling. It is a sealed lead acid battery with the built in valve. Following disconnecting the battery, testing shows the new battery to be over 7v. After 20mins or so this steadily drops to 6.6 or just below ( when the battery has settled down).

My question is, the charger appears to be the genuine thing and has an output of 6v at 0.6a. is the charger not throttling back when the battery is nearly fully charged hence the reason for the bubbling? Should i invest in a smart charger that will reduce down near full capacity.

I phoned the company today ( feber) and they stated it should have a feber battery not ultramax ( we got it with an ultrmax battery and the new replacement is an ultramax) so i’ll play safe an invest in the genuine feber battery, but dont want to pay £40 for the genuine battery to be overcharging it and burning it out.

Many thanks
Rob