That seat does not rearface, so it is not appropriate for your children.
Best practice would be to keep both of them rearfacing until 35 pounds, but if you want to FF the 3 year old, that would be acceptable. The 18 month old REALLY needs to stay Rfing until at LEAST 2 years old, poer the new AAP recommendations.
Also, that seat is really crap. It has low top slots, so most kids outgrow it before 40 lbs anyway, and then it makes a TERRIBLE booster, you woudl absolutely not want to use it as a booster. So, not a great value at all.
Some good, but cheap, spare seats are the cosco scenera, safety 1st uptown and avenue, and the evenflo triumph advance. The summit runs about $100, and the scenera is $50, the uptown/avenue is $65/80 and the EFTA can be found on sale for $120 or so most of the time. these seats all rearfaceto 35 lbs and then FF to 40, or 50 for the EFTA.
I second what the PP said..what seasts do they have in your car? It might be more cost-effective to hand down your seats to the babysitter car and get new seats for your car , depending on what seats you have.