Radio Controlled model aircraft and cars can be great fun but there are also a number of dangers lurking for the unwary such as what happens if your little model aircraft described as a 'Park flier' hits someone in the head? if you are not insured then I hope you have a deep and well filled pocket because litigation can be very expensive if you are found to bve negligent and have to cough up damages so get yourself insured preferably through a local and well organised aero club with affiliation to BMFA.
Cars are also great fun because they are cheap and can be used by kids fairly safely but again there are dangers such as Ni-Cad and Ni-Mh batteries that can leak and are quite toxic and the latest type of battery Li-Po is a Lithium Polymer battery that requires special charging and charger, they are very good at the job they do and weight half as much as a Ni-Mh but are rather prone to bursting into flames if not properly handled or charged.
Much of the above can also be related to boats using batteries but many aircraft cars and boats use internal combustion engines which require highly inflammable fuel so be very careful when filling gas tanks and so on to avoid a fireball, make sure that there is some ventilation in the fuselage or hull to disperse fumes which can explode totally destroying your model. Nitro and other simularly fueled engines also run very hot so good insulation and great care to avoid burns is a must.
There is an aweful lot of stuff available from China and Hong Kong these days and in the main it is OK for the price but remember if something arrives and is faulty then it will cost a lot of pennies to send it back for replacement and it is a cost that you the buyer will have to pay so it may not be worth the financial outlay to return an item, so better to buy a higher priced item at home where you can take it back to the shop if need be and in all probability it will have a 12 month guarantee.
There are a lot of aircraft plans around these days that are pirated or illegally copied from magazine plans and so on and in many cases as some of us have learned to our cost when a plan is enlarged from a 1:12th scale sheet up to 1:4th scale distortion and stretching of the image takes place, what is the point of paying out £20 plus another £10 P&P for something from abroad that is just not accurate and who builds at that scale anyway apart from a few very experienced builders? save your money and buy original plans from reputable dealers in the UK or US where quality can be guaranteed.
Building model aircraft in particular needs accurate plans otherwise the end result is not stable in flight and a pig to control, do you really want to spend hundreds of hours building your pride and joy model only to see it fly itself into something hard and immovable when you ask it to do something it cant do? a distorted plan means a distorted model that will never fly properly so give it a decent start in life and use original plans and decent materials.