The 2 combination seats I personally prefer are the
Recaro Young Sport & Cosco Apex.... Since your children are not obese, the RYSport would be the safer buy, although more expensive -- both have the same top harness height measurement, but the RYSport becomes a safer
booster in the long run (deeper head wings filled with energy absorbing foam like bike helmets + more extensive crash testing). Otherwise, any correctly used 5 point harness within your budget is safe & all highback boosters are about 70% safer than backless boosters in side impacts, even without the head wings: deeper head wings are helpful, though, when the vehicle lacks rear side curtain airbags.