Joseph are you asking for Israel or the states? I started a thread a while back asking for a replacement for the K9 filter (they don’t work on the latest version of android and I have the feeling they won’t be updating it anytime soon). Somebody in the CR recommended Mobicip which was a very comparable alternative. They have a filtered browser and you can block certain apps (including the appstore itself so that you can’t download new apps without the password).
The filtered browser isn’t perfect (you can still access things like google image search although I think it’s forced safesearch) but it’s definitely better than no filter at all. The app is free although to do anything more than the stuff I just listed I think you need a paid version.
If you’re asking for Israel I got a mushgach phone which I found out about recently. Basically it takes two sim cards – one kosher and one not so you have two phone numbers you can use. The phone is a smartphone and you have internet access on the non-kosher SIM but there is no internet browser, youtube, etc. There is just basic parve apps like email and weather. At the place I got mine from the guy can add on certain other apps like waze and whatsapp. As long as you don’t need to check any websites it’s great option as you don’t need to worry about any chinks in the armor of the filter since there’s no browser. It’s great for an adult, however if you’re looking for a child it’s probably more than they need.